OptiFine is a Minecraft optimization mod.

It allows Minecraft to run faster and look better with full support for HD textures and many configuration options.

The official OptiFine description is on the Minecraft Forums.

Resources: translation, documentation, issue tracker.

Social media: Discord, Twitter, Reddit.

Features

• FPS boost (examples)
  - doubling the FPS is common
  - decreases lag spikes and smooths gameplay
  
• Support for HD Textures (info)
  - HD textures and HD fonts (MCPatcher not needed)
  - custom terrain and item textures
  - animated terrain and item textures
  - custom HD Font character widths
  - custom colors
  - custom block color palettes
  - custom lighting
  - unlimited texture size
  
• Support for Shaders (info)
  - based on the Shaders Mod by Karyonix
  
• Dynamic Lights
  - allows handheld and dropped light emitting items to illuminate the objects around them
  - similar, but not related to the Dynamic Lights mod
  
• Variable Render Distance (example)
  - from Tiny to Extreme (2 x Far) in 16m steps
  - sun, moon and stars are visible in Tiny and Short distance
  
• Configurable Smooth Lighting (examples)
  - from 1% - smooth lighting without shadows
  - to 100% - smooth lighting with full shadows
  
• Performance: VSync
  Synchronizes framerate with monitor refresh rate to remove split frames and smooth gameplay
  
• Smart Advanced OpenGL
  - more efficient, less artifacts
  - Fast - faster, some artifacts still visible
  - Fancy - slower, avoids visual artifacts
  
• Fog control
  - Fog: Fancy, Fast, OFF
  - Fog start: Near, Far
  
• Mipmaps (examples)
  - Visual effect which makes distant objects look better by smoothing the texture details
  - Mipmap level - OFF, 1, 2, 3, Max
  - Mipmap type - Nearest, Linear
  
• Anisotropic Filtering (examples)
  - Restores details in mipmapped textures
  - AF level - OFF, 2, 4, 8, 16 (depends on hardware support)
  
• Antialiasing (examples)
  - Smooths jagged lines and sharp color transitions
  - AA level - OFF, 2, 4, 6, 8, 12, 16 (depends on hardware support)
  
• Better Grass
  Fixes grass blocks side texture to match surrounding grass terrain
  
• Better Snow (examples, credit)
  Fixes transparent blocks textures to match surrounding snow terrain
  
• Clear Water (examples)
  Clear, transparent water with good visibility underwater
  
• Random Mobs
  Use random mob textures if available in the texture pack
  
• Connected Textures (examples)
  Connects textures for glass, glass panes, sandstone and bookshelf blocks which are next to each other.
  
• Natural Textures (examples, idea)
  Removes the gridlike pattern created by repeating blocks of the same type.
  Uses rotated and flipped variants of the base block texture.
  
• FPS control
  - Smooth FPS - stabilizes FPS by flushing the graphics driver buffers (examples)
  - Smooth Input - fixes stuck keys, slow input and sound lag by setting correct thread priorities
  
• Chunk Loading Control
  - Load Far - loads the world chunks at distance Far, allows fast render distance switching
  - Preloaded Chunks - defines an area in which no new chunks will be loaded
  - Chunk Updates per Frame - allows for faster world loading
  - Dynamic Updates - loads more chunks per frame when the player is standing still
  
• Configurable Details
  - Clouds - Default, Fast, Fancy
  - Cloud Height - from 0% to 100%
  - Trees - Default, Fast, Fancy
  - Grass - Default, Fast, Fancy
  - Water - Default, Fast, Fancy
  - Rain and Snow - Default, Fast, Fancy
  - Sky - ON, OFF
  - Stars - ON, OFF
  - Sun & Moon - ON, OFF
  - Depth Fog - ON, OFF
  - Weather - ON, OFF
  - Swamp Colors - ON, OFF
  - Smooth Biomes - ON, OFF
  - Custom Fonts - ON, OFF
  - Custom Colors - ON, OFF
  - Show Capes - ON, OFF (supports HD capes)
  
• Configurable animations
  - Water Animated - OFF, Dynamic, ON
  - Lava Animated - OFF, Dynamic, ON
  - Fire Animated - OFF, ON
  - Portal Animated - OFF, ON
  - Redstone Animated - OFF, ON
  - Explosion Animated - OFF, ON
  - Flame Animated - OFF, ON
  - Smoke Animated - OFF, ON
  - Void Particles - OFF, ON
  - Water Particles - OFF, ON
  - Rain Splash - OFF, ON
  - Portal Particles - OFF, ON
  - Dripping Water/Lava - OFF, ON
  - Terrain Animated - OFF, ON
  - Items Animated - OFF, ON
  
• Fast Texturepack Switching
  Switch the current Texturepack without leaving the world
  
• Fullscreen Resolution
  Configurable fullscreen resolution
  
• Debug
  - Fast Debug Info - removes lagometer from debug screen
  - Debug Profiler - removes profiler from debug screen
  
• Time Control
  Default, Day Only or Night Only - works in only in Creative mode
  
• Autosave
  - Configurable Autosave interval
  - A fix for the famous Lag Spike of Death