In Starcraft, the word drop has two meanings- one is where you boot a player from a multiplayer game after they've lagged out for the requisite forty-five seconds, and is usually the result of someone getting disconnected from their ISP or from Battle.net. This usually concludes in a notch on the disconnect side on the lagger's record, and the game progresses on.
The other definition refers to the use of shuttle, dropships, or overlords to ferry units directly into your opponent's base. While this method allows you to directly bypass any defenses that your opponent may have stationed near the front of his base, it also has the drawback of making the units carried in the transport extremely vulnerable- destroy the shuttle, and all the units it carries die along with it. If your opponent suspects a small drop, he is more than likely going to be able to hold it off with static defenses and units stationed within his base. If you are able to catch him unaware, though, the payoff can be sw33t.
The most commonly-used units in the smaller drops are Reavers and Lurkers, as their splash damage dominates peon lines like no tomorrow. However, it is not uncommon to find Vultures, Zerglings, or Templar of either variety there, as they have the capacity to deal powerful damage as well to workers. Larger drops- consisting of five or more overlords or dropships- usually bear a cargo of marines, medics, siege tanks, and SCVs when fielded by a Terran player, or of Hydralisks and Zerglings if used by a Zerg player.
As to defending against a drop, the best way to stop it is to know about it. Recon, recon, recon- if you aren't doing it already, then you ought to start. If you see a number of Overlords or Dropships clustered about in one corner of an enemy's base, then fortify your defenses in your own- or, alternately, attack him elsewhere while he's organizing the attack.