A patch family has 2 meanings in this context, it specifies which changes you would like in any given patch, and also provides a mechanism for supercedence and dependency tracking. I'm not going to get into supercedence here other than to say that for each patch family in a patch will get an entry in the MsiPatchSequence table.

The PatchFamily element is a child of the Patch element. You can have as many PatchFamily elements in a Patch as you want. Under a PatchFamily element, there is a set of Reference elements that can be used to pull various items into your patch. Just as in your product authoring, referencing anything in a Fragment will result in that entire fragment being pulled in. WiX will handle file and media sequencing as well as the Sequence tables for you.

Example:

This PatchFamily would select the Fragment that contains the Component "MyComponent" and add it to the patch.

<PatchFamily Id="MyPatchFamily" supercede="yes">
    <ComponentRef "MyComponent" />
</PatchFamily>

Sidenote:
One thing to note is that there are rules about patch families. Once you ship a patch family, you must keep its references the same or add to it (grow it) but never remove items from it. All items in your product must be a part of only one patch family. When you change the build that you are targeting (re-baseline), patch families start over so you can change the contents and asociations at this time.