bases??

this might seem like a stupid question but I'm 17 and no one, I mean no one, I know uses the bases as reference for sex or anything like that. around here if you know someone is dating someone you automatically assume they kissed. or whatever you just. say "y'all fucked?" "y'all kissed" or "hey person my so and I fucked" and it gets around. 
so what exactly ARE the bases???