If you start dealing with microcontrollers, you will definitely face the term ‘pull-up’ resistor. This is not a resistor type, but rather its purpose. As name already states it pulls something up.
A1RONZO have written very explanatory tutorial where he goes through almost all aspects of pull-up resistors including typical circuits on AVR microcontrollers, how to calculate its value and how to deal with internal and external pull-ups. If you are just starting to learn interfacing and programming microcontrollers, be sure to read through and understand whey they are there.