One or more people, usually from message boards, that record themselves playing video games through screenshots or captured video (Mostly the latter). This can be any game, from Doom to Zelda: Ocarina of Time.
The most popular of the Let's Play videos come from the extremely difficult Super Mario World hack titled "Super Kaizo World". Usually Let's Play videos consist of jokes (Good, bad, and/or corny), frustration, and bewilderment by the ones playing. Some also explain gameplay, easter eggs, and general trivia pertaining to the game being played.
"I want to see some Let's Play videos."
"Have you seen that dude's LP of Super Kaizo World? Shit was hilarious!"