A logic gate is an electrical component, which takes electrical inputs and performs some logical calculation on them, a Boolean function, and then outputs a result, a single outcome.  A logic gate exists in the hundreds of millions in every modern computer and forms the building blocks for all of the operations, which are performed by processors and memories.  They can be made of a variety of materials and components, but by far the most common is a transistor-based gate.  MOSFETS are the most common because of their ease of the integrated circuit manufacturing process.

Some of the most common logic gates are the AND gate, OR gate, and Inverter gate.  Simple AND gates take two inputs and determine if they are both a logical 1.  If so, then the output of the gate is 1, else it is 0.  Simple OR gates take 2 inputs and determine if either one of them is a 1 or both.  If so, the output is a 1, else it is a 0.  In this case, 3 different inputs can output a 1, and only one input combination can output a 0.  Inverters simply take one digital value and switch it.  So a 1 for 0 or 0 for 1, it is the simplest of the gates. 

Gates are usually part of a much larger mathematical and logical system for performing complex tasks, but often a simple gate can find a use exclusively in a system