Cookies are widely used across the internet: you may have hundreds of cookies on your computer at any one time. Each browser has its own set of cookies so, if you run multiple browsers, you will have multiple sets of cookies on your computer.
In the past, cookies have had some bad press. Much was due to a general misunderstanding of what cookies are and how they work but some was due to justifiable privacy concerns relating to 3rd party tracking cookies. These cookies are used by advertising websites and track a web user across multiple sites. The good news is that all modern browsers allow you to easily block 3rd party cookies.