This is the ProTek R/C 4S 2100mAh, 20C Lithium Polymer battery pack. This pack has T-Style Ultra connectors pre-installed, and comes with a 5-pin Align/E-Flite style balance connector.
This battery will also make an excellent starter box battery for R/C Cars! You can significantly reduce the weight of your starter box, as one of these batteries will weigh less than one third of two 3600mAH NiMH packs (221g vs 802g). This will make your starter box almost 1.3 pounds lighter! Not to mention they will have increased voltage and punch, making it easier to start a tight engine.
Unlike most other Lithium Polymer batteries, these batteries can be charged at up to 2 times the rated current, or 2C! If your charger is capable, you can charge these batteries at 4.2A!