Tattoo "flash" is known as the sheets of designs that hang on the walls of most studios. Clients that don't know what they want and/or don't know that custom tattoos are possible can quickly choose a tattoo from these sheets.
It goes without saying that instead of getting a deeply personal design, anyone who selects a flash piece will be walking around with the same tattoo as thousands of other people. A tattoo artist can tattoo anything a client desires -- clients are NOT limited to the flash sheets. In addition, the quality of flash hanging in a studio has no relation whatsoever to the quality of the artists, as the artists in the shop may or may not have drawn the art on the walls. The portfolio of the artist is the most reliable way to determine the artist's style and skill level.
Don't be afraid to ask the artist for something original. This may delay the process of obtaining the tattoo, being that the artist will then take the time to draw out exactly what it is you want, but this additional step is worth it. Never settle for flash over creativity.