^ explaind weel obove.
Bare drive - you get bare drive, noting else.
Kits - Can include the 2.5->3.5 inch mounting braket. May or may not need depending on case. On occassions, I've simply made my oun, bat as other explained - they are cheap. The othe item that may be included in the kit is a "cloning, or Migrating" software.
This is good if going from say a smaller SSD to Newer SSD or a larger SSD. When Moving from a HDD -> a SSD it is recomended to do a clean install.
PS This software is no Biggy as EasyUS is FREE and does a great job.