A rainfly is the waterproof outer cover of a tent that goes over the tent body to keep out rain while letting the inner tent breathe. Together with the tent body it forms a double-wall tent. A full-coverage rainfly protects best in storms; partial flies save weight but offer less coverage.
A tent is a portable fabric shelter supported by poles, used to sleep in outdoors while protecting against wind, rain, and insects. Backpacking tents are sized by person capacity and prioritize low weight, while car-camping tents prioritize space and comfort. Key parts include the body, rainfly, poles, stakes, and often a footprint.