Three types of error in the lab exist: systematic error, random error, and gross error.
Systematic error occurs when the instrument you are using has a consistent inaccuracy. Perhaps the balance won't zero when the mass is zero, or it consistently reads a mass that is above or below the actual mass of the object.
Random error occurs when estimation in a measurement is made. One student may read a volume at 44.5 mL while another reads it at 44.6 mL. Or in determining when a reaction occurs, the observer takes a split second to react. There are natural limits to our abilities.
Gross error occurs when an actual mistake is made. The solid went through the filter paper, or a spill occurred.