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) (Matthew 1:20, John 12:8, 1 Corinthians 16:7).
On occasion our particle does not seem to introduce some clarification but merely serves to add emphasis; in the sense of "truly [such and such]" or "then [such and such]" (Matthew 27:23, John 7:41).
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.