The Builder in a Bottle is just like it sounds, a hard gel that you can use on tips, sculpt, or overlay that comes in a bottle.  No need for a gel brush. Great for fills or to create the perfect set of gel nails. 
Comes in Clear, Blush, and PRO Kit.
Don't forget the tips and forms.
If you are just getting the PRO Kit, make sure you pick up some Connect & Shine.