For example, concerning de Rham cohomology as explained on the lower part of page 5 here, one considers the vector space of all forms on a manifold M, ADR(M) in the notation of the paper (?).
The p-th de Rham cohomology is then defined as the quotient of the kernel and the image of the exterior derivative d acting on the space of p and p−1 forms respectively as
HpDR(M)=Ker(d:ApDR(M)→Ap+1DR(M))Im(d:Ap−1DR(M)→ApDR(M))
Can cohomology always be defined as the kernel divided by the image of a certain operation?
What does cohomology mean or "measure" in as simple as possible intuitive terms in the de Rham case but also more generally?
PS: I tried to read wikipedia of course, but it was (not yet?) very enlightening for me ...