These are very commonly used interchangeably, however they are different grammatically. That is used for restrictive clauses which are only concerned with one group or concept: We wondered about the cook stoves that were made of wood. i.e. only about those which were made of wood.
Conversely, which is used for non-restrictive clauses, which are concerned with the whole class of ideas: We wondered about the cook stoves, which were made of wood. i.e. about all the cook stoves. Note that which is always preceded by a comma in this case.