microcontroller

"A microcontroller (sometimes abbreviated µC, uC or MCU) is a small computer on a single integrated circuit containing a processor core, memory, and programmable input/output peripherals. " http://en.wikipedia.org/wiki/Microcontroller

In other words, it's a small/cheap computer that you can use for a specific function like monitoring the temperature of your greenhouse and sending you a text message when it gets too hot.