Hl2 has tons of mods so defining might be hard there are mods that uses its source base(or sumthin i forgot) for games such as zombie panic/insurgency/synrergy/etc so for that kinda stuff as for maps those are maps not mods mods are things thaat change modes gameplay everything (dont flame if i am somewhat wrong)
When people talk about Half Life 2 mods they're basically talking about games that use the Source engine. They aren't really modifications to Half Life 2. They're more-or-less completely new games with the same engine. In order to play them you need to have purchased a game that uses the Source engine such as Half Life 2 or Counterstrike: Source.
Here are all the mods in development and released for hl2, about 37 pages of them. I think hl2 and hl1 have the most mods created out of all the other games.