For Windows systems Internet Explorer is included and updated through the automatic update mechanism. For Macintosh Safari is included and updated through the automated app store system. While you will always have these available they are not your only choices. We recommend everyone have a second browser to use if for no other reason than to make sure a web problem isn't software based.
Internet Explorer has taken some well deserved criticism over the years but starting with version 10 really isn't bad. Safari has always been solid but can be quirky at times. Google Chrome and Firefox are both updated so often that sometimes their reliability suffers. Nonetheless, all of these browsers are pretty good. There are other choices beyond these that have some advantages too but these 4 are the mainstream browsers to be sure.