Errors and their types:

Errors can be grouped into two groups :
1 systematic error


 systematic: e.g. constant error (in all readings)
    cannot be eliminated by averaging
    error in measuring instrument B1
 
2.random errors
random: e.g. readings scattered (equally) about true value
    error due to observer
    can be eliminated by averaging
        (only if averaging not included for systematic


It is very to know these by heart as you will be given a direct question about errors in your physics AS exam.