There are lots of options here, and we’ll take them from the simplest to the most complicated. Bear in mind that every tent is a compromise between weight and comfort. And so we have our opinions about what works best.
The simplest is no tent at all. Just sleep under the stars. If it rains, or there is a lot of condensation, use your rain poncho to cover the bag. If there are bugs, use lots of repellent and get used to the buzzing. Advantages: You can’t get any lighter than this—and later in the season, with few bugs and clear weather, that’s pretty nice. Disadvantages: not so nice in rain or with bugs. And if there are other campers around, they live in your bedroom. You change your clothes in the bag, or in public.
A bivy sack—short for bivouac sack. This is how climbers sleep on a ledge as they work their way up a cliff. It’s just a lightweight shell that covers your bag and keeps the moisture out/off. You can even get one with bug netting to cover your head. Advantages: Still really light—and later in the season, with few bugs and clear weather, that’s still nice. Disadvantages: slightly better in rain and bugs. Each hiker needs one—and it won’t cover your pack. If there are other campers around, they still live in your bedroom. And expect some condensation in the bag from body heat.
A tube tent. This is just a tube of plastic, with a rope running through it. You tie the rope to a tree on each end, and climb inside. Advantages: These are still pretty darn light, and they work just fine in light rain. Some measure of privacy. Disadvantages: You need trees to pitch this tent: two of them. Wind will blow the rain right in the end of the tent. No bug protection. In high winds, these will be an adventure. But P took many trips just like this as a young man, and lived to tell the tale.
A hammock. Yep—just like in the islands. You can hang it between two trees, put a bivy sack with a bug net over yourself, and sleep high and dry. Advantages: These are still pretty darn light, and they work as well as a bivy sack, without the concern about water on the ground. You still need trees to pitch a hammock: two of them. Disadvantages: wind might just rock you to sleep, or blow you around. For those of us with back problems, the curvature of the spine might be a deal killer. And by the time you add the weight of the hammock and the bivy sack, you might just as well take a tent? Maybe not.
A tarp-tent. This is just what it sounds like: a single sheet of waterproof material that you pitch between two poles and stake out. It can offer very good protection from the rain, if it is large enough, but doesn’t offer bug protection. Very light---some are under a pound. Disadvantages are that you don’t get bug netting, and if you want something between you and the ground, you have to carry that as well. Most users just count on their sleeping pads for this. You’ll need poles or trees to set this up right.
A single-walled tent. There are lots of variations of these, and we really like the one that P made out of some old waterproof rip-stop nylon. You can make one of these for really very little money. Advantages: Weight should be in the 3-4 pound range (ours is just over 3), and they will keep you dry, and the bugs off. They will do a better job in a wind-driven rain. Some sense of privacy here—at least they can’t step on you in the dark without tripping over your guy lines. These are still pretty darn light, and they work just fine in light rain. You will need poles to pitch this, but it can be pitched anywhere you can get a tent stake into the ground. And if they are large enough, you can also get your packs out of the rain. Disadvantages: The only real problem here is that you have to choose between bug netting and warmth. Ours has walls of bug netting, which work great. But if it gets cold, it’s the same temperature inside and out. More traditional single-wall tents have very little netting, and are mainly waterproof material. And so when it is cold outside, they provide some measure of warmth. But that comes at a price---because your breath will condense in these tents, and create a lot of moisture. Which will get everything wet. Not good.
The double-walled tent. This is the classic, with an interior tent that is all bug netting, covered with a waterproof rainfly. They work very well, and can be used without the rainfly in nice weather. The larger ones (we use a three-man tent for the two of us!) also have room for your pack inside, and use a system of bent poles that allow them to be pitched anywhere, including a sand dune or a flat rock. Our experience says that in cold weather it will be 10-15 degrees warmer in the tent—at least until someone opens the zipper to get out. That’s good in cold weather. In warm weather, you may want to leave the rainfly off, because ventilation is not what these tents do best. The main disadvantage? You can see this one coming, can’t you? They weigh a lot more—more than double what a tarp tent or our little single-wall weighs. But for those shoulder seasons, they are the right choice, in our minds.
Lastly, you can spend over $500 on a four-season tent with all the trimmings. These will have very sophisticated ventilation systems and often provide extra antechambers for your packs. They are bullet-proof, and will not only keep you dry and free of bugs, but they might even keep you warmer. They can be used in snow, sand, and sleet. They weigh more, cost more, and unless you are planning to backpack in the snow, they will be hot and steamy inside. Only for pros and really serious winter amateurs, in our mind.