Difference between Priority and Severity
Both Priority and severity are the attributes of a bug which are provided in the bug report. These attributes defines how fast a bug should be fixed.

Priority:-Priority means how fast a bug should be fixed. Priority is initially set by a tester to the developer mentioning the time period to fix the bug, but usually finalized by the Project Manager . It is defined on the basis of business impact, development efforts and on some other factors.
Types of priority:-
1. Critical:- If a bug's status is marked as Critical then it should be fixed immediately. It means that some major functionality has been blocked and no further testing can be done.
2. High:- Once critical bugs are fixed then bugs with High status are needed to be fixed and should be included in the next release.
3. Medium:- Bugs categorized under this category may be fixed later and in the next release.
4. Low:- Bugs under this category may be fixed later. Bugs in this category are of generally cosmetic

Severity:- Severity is the degree of impact of a bug on software.
Types of Severity:-
1. Critical:- Bugs affecting critical functionality where further processing or workaround is not possible. This is also known as show stopper.
Example:- Unsuccessful installation or complete system failure
2. Major:- Bugs affecting major functionality where workaround is possible but not in obvious way.
Example:- A feature is not working in a module but can be done by different modules following complicated steps/ process.
3. Minor:- Bugs which affect minor functionality where workaround is carried easily.
Example:-A feature not working in a module but can be done easily from other modules.