Anything that is from an earlier era and looked upon with high regard or respect. Can be used to refer to music, clothing, language, or anything really.

Music so good it could elicit sexual arousal.

Anything that refers to a previous generation of a subject/idea/object/etc. Typically, they are highly regarded and sometimes the very thing that started it all.

from back in the day

A positive appellation referring to when things weren't flashy but empty of substance, were done by hard work, didn't pander to the lowest common denominator, and required real skill. Labour-saving devices, shortcuts that reduce quality and quitting before the task is done are not characteristics of "old school."

In reference to computer games, refers to a game that had substantial playability without flashy graphics or eye candy. Old school gamers appreciate difficult maneuvers, careful planning, and scorched earth policies.

In reference to role-playing games, old school refers to games that tested players' wits, could kill off careless characters, and required dedication and inner strength to play. Old school games didn't pander to the ideas that everyone is created equal, that all options are open to all races, that the markets were somehow free, and that a quasi-medieval society could have near 100% literacy.

See also classic.

The way things used to be done.

1) Retro
2) As Before (as something was done, said, etc.)