Feedback - Feedback occurs when a sound loop exists between an audio input (e.g. Microphone) and an audio output (e.g. Loudspeaker).
To avoid feedback automatic anti-feedback filters such as a Feedback Destroyer or Feedback Eliminator can be used.

Cardioid vs Hyper Cardioid - The most common of microphones is the cardioid microphone. The cardioid microphone got its name from its heart shaped sensitivity pattern, this means that the microphones sensitivity is primarily in front of the microphone but it can also pick up sound from around it. This makes Cardioid microphones such as the Sennheiser e840 best for vocals, where you can get away with moving around the microphone a little bit.

The Hyper Cardioid is the most sensitive directly in front of the microphone and the least sensitive at the rear of the microphone. Super Cardioid's like the Sennheiser e935 are most useful for vocals in situations where there is a lot of background noise such as loud foldbacks.

Balanced TRS Plug - A TRS plug looks just like a standard jack plug except it has two rings opposed to one. Internally a TRS connector has a "hot" (positive) tip,  a "cold" (negative) ring and the sleeve acts as the ground. The term "balanced" comes from the method of connecting each wire to an indentical impedence and load.