Musical notation is a visual representation of music and how it should be performed. Modern or standard musical notation makes use of many symbols, too many to list here. For more information see this Wikipedia article.
Common time is another way of saying 4/4 time. It is so called as it is the most common time signature is many forms of western music, including rock, country, and pop. In standard notation, it is can be symbolized by what appears to be a “c” rather than the more common four over four notation. The symbol is actually derived from a broken circle for reasons explained in this Wikipedia article.
Cut time is another way of saying 2/2 time. It is common in marches and musical theater. It has a faster “feel” than common time. In standard notation it can be symbolized by what appears to be a “c” with a vertical line down the center. For more, see this Wikipedia article.
In digital audio production, a “zipper noise” is an audible artifact caused by making changes to digital settings. It is often most pronounced when changing delay or reverb times. It is largely caused by the fact that digital adjustments are, by definition, made in steps. Analog can be a continuous change, preventing many of these artifacts.