Defect severity determines the defect criticality whereas defect priority determines the defect immediacy or urgency of repair

 1. High Severity & Low Priority: 
                                                       Suppose there is an application which generates some banking related reports weekly, monthly, quarterly & yearly by doing some calculations. In this application, there is a fault while calculating yearly report. This is a high severity fault but low priority because this fault can be fixed in the next release as a change request.

 2. Low Severity & High Priority: 
                                                        Suppose there is a spelling mistake or content issue on the homepage of BT.com website which has daily lakhs of hits all over UK. In this case, though this fault is not affecting the website or other functionalities but considering the status and popularity of the website in the competitive market it is a high priority fault.

 3. High Severity & High Priority: 
                                                        Suppose there is an application which gives some banking related reports weekly, monthly, quarterly & yearly by doing some calculations. In this application, there is a fault while calculating weekly report. This is a high severity and high priority fault because this fault will hamper the functionality of the application immediately within a week. It should be fixed urgently.

 4. Low Severity & Low Priority: 
                                                       Suppose there is a spelling mistake on the pages which has very less hits throughout the month on any website. This fault can be considered as low severity and low priority.

high severity & low priority 
This is the category and the Sub category of this is  :

high severity

> System Down
> user function down
> major

low priority 

> Minor 
> No Impact

Lets take a example... Gmail 

1. If gmail is opening and user is not able to login there
is System down Bug

2. If user can login and he is not able to access sent mail
and compose mail then its user function down

3. If everything is working and user is not able to logout
from gmail , but to close your gmail account there is
workaround ,, which is u can close ur web browser and clear
your cookies in that case that is major bug

4. If there look and feel issue like spelling mistake or
alignment which has no effect on functionality then that is
Minor or No impact bug

If you have any other query the u can mail me on my ID :

nishi.gautam@gmail.com