Shelter-half, rain fly, and leanto (or lean-to) are three terms sometimes used as names of a structure as in your picture. Scattered among images on the google-images pages (1,2,3) are some pictures like yours, although for the third term images like yours are infrequent. Also see tarp, tarp tent, and tarpaulin.
Among people who camp in the US, fly or rain fly is the accepted term for the outer layer of a tent-and-fly combination, where the fly serves as a rain shield or additional thermal barrier for the tent. Most multi-season camping tents come with a fly or offer one as an option. Flies for tents typically are shaped to roughly match a given tent (ie are not flat) and are sewn together from pieces of thin nylon fabric, often waterproofed. A tent and fly can be used in several combinations: the tent alone, in mild dry weather; the fly alone, as a quickly-set-up rain or sun or sleeping shelter; or in more-severe weather, used together, with the fly keeping the tent drier and warmer than it would otherwise be.
A tarp, on the other hand, may be made of canvas, nylon, or various combination fabrics (such as rubberized or plasticized nylon). Tarps typically are flat fabric, with grommets along the hems. A shelter-half is a particular kind of fly, such that two shelter-halves together can make up an enclosed tent. Shelter-halves have been common items in surplus stores after various wars, and while (in my opinion) they are practically obsolete, the term shelter-half lives on.