Irrlicht Engine

I have couple of questions which may be for many of you guys here trivial, but i am new to 3d etc.:

1) What is a mesh ? What exactly does it contain, etc.
2) Is it possible to build a mesh "on the fly" (with the api) ?
I mean there are functions which can import levels (bsp, etc.) and build
meshes, but i am more interested in building meshes in the code.

1.) Basicly a Mesh is a couple of vertices ( just a point with position, normals, color etc) and indices connecting the vertices to triangles which can be rendered. Mostly they have information their about materials ( textures to map on, how to "react" to lighhing". And in irrlicht they have bounding boxes.
These datas are enough to use Mesh for terrain, maps, charakters and whatever you like.
2.) To create a Mesh "on the fly" you just need to create these kind of datas, so it is possible to create a mesh in the code.

thanks for the answer.
After reading a bit through the forum, i found that meshes could be
created with smesh and smeshbuffer.
If i understood it correctly Meshbuffers are like 'polygons' and meshs like
'objects' which consist of meshbuffers.
I think some sort of tutorials on meshes and meshbuffers would be great for beginners. External mesh loading are nice, but to become more familar with the engine , manipulating meshes and meshbuffers help a lot more.