I'm struggling to get my head around the amount of combinations I can have from 6 letters, ranging from a single letter upto 6 letters. They mustn't be the same combination twice such as abcdef is the same as acdfeb.

the letters are a, b, c, d, e, f

i know the obvious ones like

a

b

c

d

e

f

ab

ac

ad

ae

af

bc

bd

be

bf

and so on, can someone help with all the possible combinations or know a quick way to work them all out, thanks in advance if you can help