Both are used interchangably, but one could say that trash is refuse that hasn't yet been placed in the bin (ex: "That broken pen is trash. Get rid if it." Or "Ew! Throw that tissue in the trash can."), while garbage is associated with refuse that has already been thrown out (ex: "The garabage [or garbage cans] are out on the curb waiting to be hauled away in the morning." Or "I just cleaned out the garage, so now I have a box of garbage to carry to the dump.")

Both are used equally often.

I would say they are both used quite often. Personally, I have used both words.