Mod is a dandified youth subculture that developed and was strongest in the 1960's, but has experienced many reincarnations since then and is still very much in existance.
As with many subcultures, the defining characteristics are clothing and music, both of which become an obsession for Mods. The clothing has a vintage, well-taylored, and peacockish look, favoring bright patterns, plenty of color, brands like Ben Sherman and Fred Perry, and slim-cut, three button suits (preferrably Italian) in the style of the sixties. The favored music of Mods is varied rather widely but, for the most part, shares the characteristic of being danceable. Favored genres include old school Ska, Soul, and R&B as well as New Wave, Britpop, and some things that, today, are heaped into the rather loose label of "Alternative Rock".
Another defining characteristic of Mods is their chosen mode of transportation, the Italian moterscooter (most notably Vespa and Lambretta). Not everyone who rides a scooter is Mod, however. These scooters tend to be in a retro style and sometimes heavily customized.
Unlike the case with some other social subgroups, there exists no Mod ideology or philosophy(outside of a general avoidance of violence because of unwillingness to damage their clothes and the idea that, as The Who put it in one of their earliest songs "you gotta be cool"). However, this does not mean that Mods do not have any depth; only that there is no specific mindset associated with the group.
Mod and Emo have almost nothing to do with each other.