Clock accuracy describes how well the actual frequency of the clock matches the specified frequency. An oscillator generates a clock. However, an oscillator never generates a perfect frequency. The accuracy of the oscillator-generated clock is affected by the quality of the crystal and the oscillator's assembly.
You can describe timing errors in several different ways. Some common units of timing error are parts per million (ppm) and parts per billion (ppb). Parts per million gives you a fractional value of error. For example, to find the error in Hertz of an 80 MHz oscillator with 5 ppm error, you multiply the frequency of the oscillator—80,000,000—by 5 divided by 1,000,000 or [80,000,000 Hz (5 Hz/1,000,000 Hz) = 400 Hz].
From this equation, you see that the oscillator can be off by as much as 400 Hz. Therefore, the actual frequency of the oscillator can be anywhere between 79,999,600 Hz and 80,000,400 Hz. Parts per billion is similar to parts per million, and it is used to describe more accurate clocks.