A SNP is a single nucleotide polymorphisms. That means that it is a single small change in your DNA code. These changes are rare. Once they happen, they seldom change back (back mutate).
An STR is a short tandem repeat. This is a place in your DNA code where a letter sequence is repeated. For example, AGTAAGTAAGTA is three repeats of the sequence AGTA. STRs have a fast mutation rate. When they change, it is an increase or decrease in the number of repeats. STR values changing back (back mutate) are more common.