Usually you have two types of "formats":
The first category are the ones that are either built for data exchange (e.g. collada, FBX) or simply are source assets (like .obj) that can be read/written by different modeling tools. Many times those are verbose human-readable (text/XML) formats too. Generally these source assets need to be processed (triangulated...normals calculated...etc) in some manner and/or are slow to load because you're parsing XML or text rather than binary data.
The second category is the format that usually source assets get compiled into, and usually are binary and specific to your application/game. The idea is to do offline-compiling of source assets into this type of format so loading models at runtime would be very fast (e.g. compact binary data that is very close to the in-memory representation of your models).
For the Direct3D or OpenGL API's, there is no set "model format"...you're simply dealing with buffers of primitives (lines, triangles, etc), however you organize those are completely up to you. It can be as simple as each mesh piece is just a vertex buffer and an index buffer.
There exist libraries out there to do model importing into a general data structure that you then can translate to your own internal format, one such library that you can look at is the Assimp C/C++ library. If you're working in C#, you can take a look at the AssimpNet wrapper too.
Edited by Starnick, 30 March 2013 - 01:34 PM.