Errors of various kinds are unavoidable in technical environments. However, in these environments, an error isn't necessarily the same as a mistake. The term is sometimes used to refer to the normal expected variation in a process. Being able to differentiate between random and systematic errors is helpful because systematic errors normally need to be spotted and corrected as soon as possible.
Random errors in science are the usual, everyday variations that are expected and anticipated in certain processes. They're often associated with measurements and transfers. For example, if someone is filling a calibrated beaker up to a mark with water, they won't always add precisely the same amount. Sometimes they'll add slightly more, sometimes slightly less. Such errors can be reduced by using machinery to perform tasks instead of humans, but they can never be completely eliminated.
Random Error Characteristics
Random errors have several common characteristics. In a well-run process, they're typically quite small and may often be small enough to be inconsequential to that particular process. As well, as the name suggests, they're truly random. This means that the error will sometimes be on the high side, sometimes on the low side and there will be an approximately equal number of misses in each direction over a reasonable time frame. The direction of the miss, high or low, will also be random and unpredictable and the magnitude of the error may vary slightly from time to time.
Systematic errors are also sometimes called bias or special cause errors. These types of errors are always in one direction. For example, a machine with a systematic error dispensing a liquid may always dispense a volume that's below the desired amount. Rather than being due to normal, expected variation, as are random errors, systematic errors are generally due to a problem of some sort. The problems causing systematic errors can range from faulty equipment to a poorly trained employee.
Systematic Error Characteristics
Systematic errors have properties which are quite different from random errors. They tend to have a sudden onset, since they're often due to a machine or process failure. So a process which is working one day and not the next may be exhibiting systematic errors. As well, systematic errors often, although not always, tend to be on a larger scale than random errors. Finally, they'll always occur as either high or low, rather than as a mixture of the two, and the magnitude of the error will usually be consistent.