Fellatio is a term for oral stimulation of the penis, or oral sex. In slang, fellatio is commonly known as a blow job.
Fellatio is an unusual-sounding, technical word that most people wouldn't use when sitting around talking about sex with their friends. Almost everyone calls mouth-to-penis sex a blow job, even though that term is slangy and vulgar. If you really have to talk about a blow job in a formal setting or in front of your grandma, fellatio is probably the word you should use.
