Phagocytes detect all 'foreign' things in your body (things the shouldn't be there, for example pathogens). Then they engulf these pathogens and digest them.
NOTE: its not just pathogens the will 'eat', they will destroy anything that is not meant to be there. They are non-specific.
Lymphocytes
Every type of pathogen has different molecules on its surface (aka antigens). When a lymphocyte comes across an antigen that isn't recognised (a 'foreign' antigen), the lymphocyte will produce antibodies. These antigens 'lock' onto the pathogens, this 'marks' them, so the phagocytes know they are to be destroyed.
Some lymphocytes stay in the blood and become memory cells. These remember the shape of the antigen needed for that pathogen. This means that, should that infection come back, the lymphocytes will be quick to destroy the pathogens as they already know the shape of the antigen that needs to be made - this is why you are immune to most diseases you've already had before, e.g. if you get chicken pox it's very unlikely you will get it again (as the memory cells remember the shape of the antigen so can quickly and efficiently make antibodies).
photos credit: leavingbio.net
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