Rybka 3.0 - je defaultní motor
Rybka 960 - je motor pro hraní fisherových šachů (odtud název 960 možných začátečních pozic)

Co se týče zbýlých dvou, je to dobře výsvětleno na homepage Rybky.

What are the "Human" and "Dynamic" versions?
These are special Rybka versions which evaluate positions differently. The Human version gives evaluations (and plays moves) which are more consistent with human grandmaster practice. The Dynamic version evaluates more speculatively, placing a higher emphasis on dynamic factors and a lower emphasis on static ones.

What are "static" and "dynamic" factors, and how does Rybka classify them?
Static features are ones that are unlikely to change quickly. Examples include material advantage, better pawn structure, advanced passed pawns, and better combination of pieces on the board (such as bishop pair). Dynamic features are ones that can change very quickly, such as lead in development, better mobility, and more pieces attacking the enemy king. These definitions are in various chess books. Some features are in between, and it is a matter of opinion as to which class they should be put into.