Terminology
Note that a free abelian group is not a free group except in two cases: a free abelian group having an empty basis (rank 0, giving the trivial group) or having just 1 element in the basis (rank 1, giving the infinite cyclic group). Other abelian groups are not free groups because in free groups ab must be different from ba if a and b are different elements of the basis, while in free abelian groups they must be identical. Free groups are the free objects in the category of groups, that is, the "most general" or "least constrained" groups with a given number of generators, whereas free abelian groups are the free objects in the category of abelian groups; in the general category of groups, it is an added constraint to demand that ab = ba, whereas this is a necessary property in the category of abelian groups.
Read more about this topic: Free Abelian Group