Severity:rates the bug as either it belongs to major,minor 
or trivial.
Priority:rates comes under high,medium low.
ex: part   severity      priority
    a         major       medium
    b         minor       high
    c         trivial     low
always we have to work on bugs based on priority wise only.
because of priority only we can clear other major issues 
since one of minor issues may affect the working of other 
major parts.and also the client may analyse the issue and 
change the requirement.some times even the severity is low 
if the priority is high we have to work on that coz it 
might be waiting for delivery.
here we need to concentrate on part b in the given ex.

Severity:the seriousness of the defect in terms of the 
functionality is known as Severity.
Critical:not able to continue testing without resolving the 
defect
Ex:application crash, database corruption
Major:able to continue but mandatory to resolve
Minor:able to continue testing but may or may not resolve


Priority:the importance of the defect in terms of customer 
is known as Priority.

depending on the severity of the defect the priority is 
given.

Either high, medium or low.

Priority:the importance of defect fixing in terms of 
customer is known as Priority.
High: for critical defects
Moderate: for major defects
Low: for minor defects

Thank you