The letters are which version of the 802.11 standards it supports.
802.11b and 802.11g are 2.4GHz-only, with b being up to 11Mb/s and g being up to 54Mb/s. Both obsolete.
802.11a is 5GHz-only, 54Mb/s. If it says it supports a, then you know it's 5GHz-capable. Obsolete.
802.11n is both 2.4 and 5GHz, though cheap cards often only support 2.4GHz, Speed depends on how many antennas it supports, with 150Mb/s per spatial stream (one spatial stream per antenna) per band. Most routers support two spatial streams only, though some three and really cheap ones only one. Superseded, but still very common.
802.11ac (they ran out of single letters) is 5GHz only; it falls back to 11n speeds for 2.4GHz. More streams is better; don't pick anything under 2x2 IMHO. You're unlikely to have a router with support for more than two spacial streams (written 2x2), though, so more is likely a waste of cash.
They're all backwards compatible, but will work at the speed of the slower device.