'wenn' means 'if' ( or when), and 'falls' means 'only if'. This means the condition in the if-sentence is much stronger, for example:
Wenn/Falls es regnet, gehe ich nicht spazieren.
In the wenn-sentence, the rain is one condition (among others). In the falls-sentence, it is the only condition.
In general, you can always use 'wenn', but not always 'falls'.