One of the menu options under the Contact Overview menu that is accessible when within a contact record is the Know Your Client section of the contact record:
After clicking Know Your Client, you'll then see the Know Your Client area for the contact and can begin filling in your applicable information for them.
Here, let's look at the Keywords section of this page:
If no data has been entered here, it will be blank as above. To add data, just click the Edit button in the top right corner of this section. You'll then see the below:
If you click the arrow at the far end of the box where it says "Start typing..." you'll see all available Keywords within your database and can click one to select it. Or, you can begin typing a Keyword and any that are available and match what you are typing will be available for selection (or, you can add a new Keyword to your database). After making your selection, simply click Add Contact Keyword. The Edit Contact Keywords box will remain open, allowing you to make additional selections if desired. Otherwise you can click the X in the top right corner of the box to close this out. This area might then look similar to the below:
After adding a Keyword, if at any time you want to delete it from a contact's record, you can hover over it here and you'll see a little icon to its left:
If you click this, you'll get a popup asking if you want to delete. If you answer in the affirmative, it will remove the Keyword from the contact.
Keyword is a searchable field from your Advanced Search page.
Also, there is a Keyword Report accessible from the Reports menu on your Advanced Search page.
Custom Exports also allow you to add any desired Keywords to the export.