combinating variables

hi everybody.. more like a math question but: I have 6 variables named a,b,c,d,e,f. i wanted to know in how many combinations can i possibly match them together? and is there a way to display the possible combinations easily? for example:
a & b
a & c
a & d
a & e
a & f
a & b & c
a & b & d
... and so on ...