A flat package has a very small package size. The connection wires are on the opposite long sides.
Flat packages have a small footprint and are surface mountable, i.e. designed for SMT technology. They are available in ceramic and plastic packages, in extremely flat design, with different connection arrangements or with bumps on the package bottom side. Examples: BQFP, CQFP, LQFP, PQFP, TQFP.
The situation is different for passive electronic components. These can be designed for through-hole technology but also for SMT technology.