What is the difference between `踢足球` and `踢球`?
They have the same meaning on usual---playing football. For 踢球's 球 ,it is the football in general. If we must point out the difference,there is a tiny difference merely to 踢足球 is a specific(or definite) statement, that is to say, it is more concrete. 
When you make sentence or want to express play football, both of them are ok.:)