you can retrieve them. when you put something in the trash, nothing really changes on the hard drive. when you delete the trash, it just removes the OS's info of where that data is located. if you unplug that hard drive and plug it into another computer and run a data recovery utility, you could recover that data, since the 1's and 0's of your data is still there.
the way to make sure nobody can recover your data is to securely empty your trash (google how to do that).
and to extend that idea, you can erase the free space on your hard drive using disk utility... you can do the 7x write-over (military-grade), or 35x (if youre really paranoid). that just writes random patterns of 1's and 0's to your hard drive that many times. it takes a long time, but it's pretty good. keep in mind that even after all that, someone REALLY REALLY determined with unlimited resources MIGHT be able to recover at least parts of your data. but it's highly unlikely.