Tents are structures made from fabric that provide an enclosed space which protects the occupants from environment conditions. Historically tents were made from canvas, but now are typically made from nylon or polyester with some some of waterproof treatment. Most tents are double walled, though single walled tents are still in use. There are a number of other types of portable shelters.

Single-wall Tents

There are two types of single wall tents. The first is made using waterproof (or water resistant) breathable materials. These are typically free standing shelters designed for mountaineering, and have very similar designs such as those from Bibler, Rab, or Black Diamond. I like these sorts of shelters a lot cold, winter conditions, with low humidity. The second type uses waterproof (non-breathable fabrics) and relies on ventilation to keep condensations down. I have yet to find a shelter like this that I like, I think people would often be better served using an ultralight shelter.

• Integral Designs MK1lite eVENT: 27 sq ft, 3 lb 6oz, $530. A bombproof single-wall mountaineering tent for one (two in an emergency or if you are friendly). Could be event lighter if you use replacement carbon fiber poles (though there have been questions about whether these will stand up to severe wind… I would recommend sticking with the provided poles). I think this is the best solo winter tent ever made. Alas, it was been discontinued (except for industrial purchase) due to concerns of lawsuits since eVENT isn’t fire resistant. Other good alternatives are the slightly less breathable and heavier Integral Designs Mk1 which uses Tegraltex, the Bibler i-tent made with ToddTex, or the BD Firstlight made from EPIC.
• Black Diamond Superlight Series including a number of models including the nice two person Lighthouse: EPIC canopy/sil nylon floor adaptation of Bibler designs. A bit more breathable than ToddTex and field reports suggest that it is surprisingly water resistant, but it will wet through in an extended storm. I won’t want to use a EPIC tent in a multi-day rain storm.
  

MSR and Nemo are making a number of single walled mountaineering tents I have no personal experience with.

Conventional Double-Walled Tents

Most people seem to use double-walled tents. Tents give many people a sense of security by providing privacy and a barrier against wind, rain, insects and other small creatures. Having a double wall means that there is a barrier to protect you from brushing against the condensation which will often form on the fly. Double-walled tents that use fabric for the inner body (rather than mesh) are warmer that other shelters because they air between the inner tent and the fly can act as an insulation layer. The downside is most double-walled tents are heavier, can have ventilation problems when the fly is fully shut, and you lose a visual connection to your environment. Most US designed double wall shelters require the inner tent to be set up, and then the fly to be deployed. In a strong rain, this means you can soak your inner tent. Many of the tents from Europe and Australia have a more sensible design that lets you pitch the full structure at once, or pitch the fly and then sent up the inner tent from the inside. See Roger Caffin’s rather harsh, but generally appropriate, Shelter FAQ. While a true tent taxonomy such as found in The Complete Walker IV divides tents into a large number of categories, I will talk about two, very broad categories.

Free Standing Tents

The most common double walled tents are free standing wedge and domes. People like these because they tend to be easy to set up, can be moved around, and work well in locations where getting stakes to hold is difficult. The downsides are that they tend to be heavier, don’t ventilate as well, that most of the time you really need to stake down one of these shelter for strength and to prevent the tent from blowing away even if you have gear in them (e.g. freestanding isn’t really true).  There are a lot of other good quality 2 – 4 man free standing tents. I used to consistently prefer Sierra Designs double wall tents to their competitors. For many years, SD really seems to be one of the most innovative main stream companies, makes good trade-offs, and has nice design features. My family has owned several SD tents since the 1970s. We have love each of these tents. These days I think SD still makes good tents, but I don’t immediately assume they will have the best of class. These days Slingfin seems to be making some interesting tents. Popular light weight free standing shelter that I would suggest looking at include:

• Big Agnes Fly Creek: Would recommend avoiding unless you like cramped space and only encounter moderate weather.
• Big Agnes  Copper Spur available at a variety of weights / price points. Usable in moderate three season conditions. Light but somewhat fragile.
• Slingfin Portal: has gotten some excellent reviews as a light weight 2 person, free-standing shelter which can stand up to 3+ season weather.
• Big Sky Tents: makes some of the lightest, free standing shelters on the market. I would recommend carefully checking what their backlog is before giving these folks business. In the past people have waited months to get their orders delivered.
• The MSR Hubba (1 person), HubbaHubba (2 person) One of the most popular light weight free standing shelters. More roomy that the Seedhouse, better vestibules than the SD Lightning. Not quite as stable in the wind as the Lightning. I have only check this tent out in stores, I have no field experience. Friends have reported that extra space costs strength in a storm, especially when facing high winds, but they have been happy with this tent. 
• REI Quarter Dome (T1, T2, T3) seem to be good, light weight, 3 season, free standard shelters. My friends original 2 person quarter dome wasn’t as stable or well made as my Lightning, it was cheaper and had two vestibules.

Tunnel / Arch Tents

Tunnel shelters typically have 2 or 3 arches that provide support. These tents need to be staked to the ground, but often you can get away with 4 stakes except in harsh conditions.  Tunnel tents tend to be longer than an equivalent dome, and require a fairly flat space to get adequately taut pitch.  A well pitched tunnel tent can survive very harsh conditions. If similar materials are used, a tunnel tent tends to be light weight than a dome. Tunnel tents tend to be designed with good ventilation options. Higher quality tunnels designed to handle nasty conditions tend to use equally sized arches. I would recommend staying away from any tunnel tent which uses fiberglass poles since both the poles and the tent are most likely not built to handle harsh conditions.

• Hilleberg – Make a variety of tunnel tents. Most are designed to stand up to Northern Europe’s winters. Very well made.
• Stephenson’s Warmlite 2RSW (2 man 42sq ft, 3lb 4oz, <$500) has been one of the lightest tents on the market for years. Warm, very stable in high winds, and handles snow well provided pitched tautly. The vestibules having a floor was a bit annoying because when you enter and exit in the rain/snow because some gets in and doesn’t have anywhere to go. Should note that while it has two walls, the inner wall is not breathable which makes it different from nearly everyone else’s double wall. My experience is that condensation on the inner wall was less than other non breathable single wall shelters.

Other Tent Designs

There are a number of light weight shelters that use a single hoop, or trekking poles to form a ridge line, much like some of the ultra light shelters listed above. In general these shelters are not up to facing extreme weather conditions, but they can provide light weight shelter for 3 season conditions many places (4 in places where winters aren’t too harsh).

• Hilleberg Akto is quite light for a solo four season tent, and is designed to put the fly up first followed by the tent body which is really nice if you are setting up in the rain. Huge amount of space under the vestibule. The tent is a little short for my taste because I can’t sit up without bumping by head and doesn’t handle heavy snow loads well. On the other hand, everyone I have talked to who owns this tent loves it.
• Terra Nova.Laserlite is a 1+ person tent similar in design to the Akto but weights a mere 34oz including poles while providing more headroom.
• Tarptent Scarp1 1 person, 2.75lb and Scarp2 1 person, 3.25lb double walled, winter shelters which is somewhat similar to the Hilleberg Akto. An extra 12oz for a pair of poles makes it free-standing.
• Montbell single pole tents… pole runs across the long axis rather than the short favored by most of the other monopoles.