What is a filter and how do I create one in Gmail?

Filters are used to automatically manage emails as you receive them.  You can define a set of criteria and then select the action you want Gmail to perform when you receive emails matching the defined criteria:

1. Click the gear icon located in the upper-right portion of your Gmail screen
2. Select Settings
   
3. Select the Filters and Blocked Addresses tab
   
4. Select Create a New Filter
   
5. Enter the criteria desired for the filter
6. Click Create filter with this search in the bottom right
   
7. Mark the desired actions and click Create filter
           a.  If you want the filter to be applied to existing messages that match your criteria, 
                be sure to click the checkbox that says Also apply filter to matching conversations