A tick is one cycle of the game loop.
The game loop runs at 20 ticks per second, so one tick counts as 1⁄20 of a second, or 0.05 seconds. An in-game day lasts 24,000 ticks or 20 minutes. Ticks do not affect the game's FPS.
A redstone tick spans the length of two game ticks, so one redstone tick counts as 1⁄10 of a second, or 0.1 seconds. By default, a redstone line doesn't take any time to travel, but a redstone repeater increases the delay by 1-4 ticks, depending on what it is set to.