By default, Minecraft is limited to rendering blocks at 16 pixels by 16 pixels. This mod adds support for HD textures; 32, 128, 256, even 512 pixels per block. It also adds a lot of options for better looks and better performance. Doubling your FPS (frames per second) is quite common.
You can download OptiFine here.
Features FPS boost - commonly doubles user's fps - makes for less lag spikes and smoother gameplay
Supports HD Textures - HD textures and HD fonts - custom terrain and item textures
Configurable Smooth Lighting - from 1% - smooth lighting without shadows - to 100% - smooth lighting with full shadows
Option to zoom - Default is LCTRL - Useful for spotting players in the distance.