Full disclosure: I hike in the desert most of the time so I don't deal with rain or bugs.

Tents are useful for heavy rain, cold weather, or bugs. Tarps are useful for drier climates when rain is not in the forecast and bugs are not a problem.

Tarps are problematic in the desert (no trees) or high-use areas (not allowed to tie things off to trees). In the desert, all I carry is a light-weight tarp and don't even pitch it unless it is raining. Hiking sticks can be used to rig a tarp.

Tarp tents are for light weight backpackers. They usually have to be rigged with hiking poles and many, many tent stakes. They are one-layer tents.

Two-layer tents have an inside structure which is not waterproof and an outside rain fly. This allows sweat to travel through the inside of the tent and collect on the fly. This works in theory better than it does in practice. In a bad storm, equipment in the tent often gets damp with sweat condensing on the walls.

Free standing tents have over-arching poles which hold the tend up without stakes. Stakes are still advised because of wind. Free standing tents are good for sand, rock, or areas above treeline.

A two person tent usually holds one and a half persons. Tall persons are advised to try out the tent they wish by lying inside to be sure the head and feet will not crowd up against the tent material and collect sweat.

There are ultra light pancho/tarps which are designed to be used as raingear and shelter. The problem being that once the tarp is pitched, the raingear is in use. This means one cannot leave the tarp if it is raining.