Libraries provide extra functionality for use in sketches, e.g. working with hardware or manipulating data. To use a library in a sketch, select it from Sketch > Import Library.

• EEPROM - reading and writing to “permanent” storage
• Ethernet - for connecting to the internet using the Arduino Ethernet Shield
• Firmata - for communicating with applications on the computer using a standard serial protocol.
• LiquidCrystal - for controlling liquid crystal displays (LCDs)
• SD - for reading and writing SD cards
• Servo - for controlling servo motors
• SPI - for communicating with devices using the Serial Peripheral Interface (SPI) Bus
• SoftwareSerial - for serial communication on any digital pins
• Stepper - for controlling stepper motors
• Wire - Two Wire Interface (TWI/I2C) for sending and receiving data over a net of devices or sensors.
• The Matrix and Sprite libraries are no longer part of the core distribution.

• Keyboard - Send keystrokes to an attached computer.
• Mouse - Control cursor movement on a connected computer.