To select a suitable camping tent you should consider the conditions you're likely to encounter, the number of people you need to accommodate, and how much bulk and weight you are willing to carry.
Three-season tents are designed to offer good ventilation in spring, summer, and fall, and provide sturdy weather protection in everything but heavy snowfalls and very high winds. Many three-season tents have mesh inner bodies, which reduce condensation, and can often be used without the fly for cool, bug-proof shelter on hot nights. Three-season tents are airier, less expensive, and also lighter, more compact or roomier than four-season tents. Their versatility makes them popular with backpackers, paddlers, and cyclists.
Four-season tents are built to protect you in heavy weather, with heavier, usually more numerous, poles, and low, curved shapes to shed high winds and reduce snow build-up. Extra guy-points and lines give you more staking options. Fabrics tend to be heavier, with thicker waterproof coatings that make them more weatherproof, but less well-ventilated, and more susceptible to interior condensation. All this additional protection means greater weight and packed size, and may be overkill for anything other than ski touring, winter camping, or mountaineering.
Single wall tents are suitable for expedition and four-season use. Their 'no fly' design means their per-person weight is comparable to a bivy sack. With no separate inner canopy, they offer more headroom and useable floor area for any given exterior size. Single wall tents are made of waterproof-breathable fabrics, so they function best in the cooler, drier conditions found above the snowline, and not so well in the heat or high humidity of summer or sea-level locations.
If you crawl into a tent at bedtime and crawl out again in the morning, a two-person tent fits two. But if you end up spending days inside due to bad weather, consider a camping tent built for one or two more than the actual number of occupants.
Free standing tents assumes their basic shape (usually something dome-like) once you add the poles. This design allows you to pick them up to adjust the position or shake out accumulated dirt. Free standing tents usually provide more usable floor space and overhead room than tunnel tents. With more poles to brace them, they are usually quieter in wind and less prone to swaying. However, even free standing tents must still be pegged out for proper ventilation, and to prevent them from being blown away.
Tunnel tents hang from hoop or arch-shaped poles, and need to have the ends pegged out to form their shape. They are lighter and less bulky than free standing tents of similar size. In mild conditions, they require as few as three pegs or anchor points; in rougher weather, they can be braced with additional guylines and anchors.
Two tents with similar area will feel different depending on how the space is layed out. Tunnel tents have efficient, elongated floor shapes; dome shapes provide more room for groups to sit and socialize while waiting out storms. Steep walls provide more useable floor space and shed rain well, but catch the wind.
Tent weights are described as “minimum weight” and “packaged weight.” The minimum weight includes the tent and frame, and the fewest pegs and guylines necessary to properly set up the tent. Packaged weight includes the full tent, instructions, stuff sacks, repair swatches, plus all guylines and pegs. Conditions permitting, you can save many grams by leaving some pegs and components at home, and improvising with materials available at the your campsite.
The further you carry your tent, the more carefully you should consider its weight. For short, occasional portages or day trips centered around a basecamp, you might go with a bigger, heavier model. If you carry your tent all day, every day, you'll definitely want a lighter shelter. Features like dual doors and vestibules add a degree of comfort, but also add weight, so you'll have to decide which ones are most important to you.