When working in the Wolfram Language, you will often find it useful to view groups of functions that relate to a specific subject area or set of tasks. The Documentation Center includes guide pages and the function navigator for this purpose.
The See Also section lists links to functions that are related to the current function being viewed (in this case Join):
Guide pages contain links to function pages that share some common functionality. Click here to view the List Manipulation guide page (doing so will navigate away from this page). Much like function pages, guide pages also contain links to related tutorials and guides near the bottom of each page.
If you search the Wolfram Documentation Center, guide pages are indicated with an orange icon next to the search result. Here, the List Manipulation guide page is shown in a search result:
To browse guide pages by functional category, simply use the Documentation Center home page. Clicking a category on the home page will drop down a list containing a few guide pages that belong in that category. Here, the Data Manipulation category was clicked: