The program uses the Google protocol, meaning you must be connected to the Internet to get your results.
gTrans is compatible with 12 different language combinations, although it works mainly with English and Spanish. You'll be able to use combinations like English-Spanish and vise versa, English-French, German-English, etc.
gTrans includes additional features that allow you to make the interface transparent, keep the program on top of your other windows, etc.
The program is especially useful for translating individual words though it's less useful with longer texts due to the lack of grammatical rules.