Primary, secondary and tertiary

There are a large variety of haloalkanes.  It is useful to classify these as primary, secondary or tertiary according to the number of substitutents (groups other than H) at the carbon bearing halogen. 
Examples of these are given below.
 
classification number of carbons at C bearing halogen example
primary one
CH3CH2Br
 
secondary two
 
tertiary three

These classifications are very useful because haloalkanes having the same classification react in a similar way.  Similar classifications are used for alcohols.