The Raspberry Pi has 26 pins that can be used for I/O
On those pins it supports 3 different communication protocols, SPI, I2C, serial UART, and you can use all the pins individually for your own functions.
Here are the deep details on what the Raspberry Pi calls GPIO pins:
http://elinux.org/Rpi_Low-level_peripherals
There are no built in Analog to Digital (ADC) pins, but there are add on boards that do ADC:
http://elinux.org/RPi_Expansion_Boards# ... and_RTC_Pi
The ADC Pi board there is a pure ADC board with 8 pins and it works well.