Unit Insert Molds:
Mold inserts that utilize a standard, common frame. The molds mount into the frame by a variety of methods. The frame typically stays mounted in the molding machine. There are a few standards for unit inserts, Master Unit Die, Round Mate, are two common ones.
The advantage of unit inserts is lower cost and typically faster build time. The disadvantage is that to move the mold to another molder it may be necessary to purchase the frame and thus negating the original cost savings.
Make sure when purchasing a unit insert that you are getting a standard one and not a custom or proprietary insert.
Free Stand Molds:
Free stand molds are molds with all features required to run on a standard molding press unlike unit inserts that have a separate mold frame.