Abarim Publications' online Biblical Greek Dictionary
The particle γαρ (gar) means because or for. It works pretty much the same as the English equivalents and usually ties two (or more) clauses or statements together in which one explains or affirms the other(s). As such it differs from υπερ (huper), which means "for" in the sense of utilization or servitude.
Our particle also often teams up with other particles to form regular expressions (for behold, for also, for neither), which can usually be interpreted without much difficulty.
The particle γαρ (gar) occurs a total of 1061 times in the New Testament; see full concordance.