Besides vs Beside

“Besides” can mean “in addition to” as in “besides the puppy chow, Spot scarfed up the filet mignon I was going to serve for dinner.”

“Beside,” in contrast, usually means “next to.” “I sat beside Cheryl all evening, but she kept talking to Jerry instead.” Using “beside” for “besides,” won’t usually get you in trouble; but using “besides” when you mean "next to” will.


Source : http://www.wsu.edu