Custom ROMs can be whatever the developer decides to put in them.
It can be a slightly modified version of the official stock OS with bloat removed for example.
It may have a 3rd party launcher or other software packed in.
Custom Mods to enhance your options with advanced power menu options (shut down, reboot, boot into recovery ). Other mods like custom battery displays and numerous other status bar changes.
Best to do is read over carefully what the specs are for each ROM which any good dev will have a full description of in his post.
You can always install your launcher of choice over the top of whatever the dev is offering.