Arduino has many low cost microprocessors.( Uno, Mega, Due. . . )
The Arduino Home Site can be found at:
Arduino is a nice simple solution for many C programming ( C++ if you want ) automation projects, so we will begin with this.
There are many different Arduino boards depending on your desires.
Arduino is a microcontroller with limited memory ( Uno 32k ,Mega 256K, Due 512K).
The I/O is limited, but sufficient if you think of it as a dedicated sub process.
The Analog inputs have a 10 bit (1024) DAQ for input and the PWM output can be filtered for a near analog constant value.
Digital inputs are made for ( 0-5 VDC for Uno & Mega 0-3.3VDC Due) and Digital Outputs can all be ON/OFF limited 40mA current with a PWM output option for several designated outputs.
A great starting point for anyone learning Arduino: