This is a rechargeable Li-ion Cell in an "18650" form factor. Made by Tenergy, this cell is rated at 3.7V and 2600 mAh. This is a "protected" type of lithium cell, with a built in PCB.
Ideal for use with the Li-ion Power Pack/Charger - 2 Cell (#28986). Catalog number: 45278. Brand: Tinkering Supplies. Brand's Part Number: H04527-01B.