Lithium-ion batteries are the right power supplies for many kinds of projects. With the right type of coupling in parallel or series, you could get the necessary voltage and mAh and power your projects. 18650 is the size of the battery cell. It means that the dimensions are 18mmx65mm.
WARNING: It may be dangerous for lithium-ion batteries to be used by individuals with insufficent information. When used incorrectly, lithium-ion batteries may explode and cause a fire, which is why they should be used with caution. Lithium-ion batteries must be charged with the suitable chargers. Lithium-ion batteries may explode if they are charged with an unsuitable charger. So, use only chargers designed for lithium-ion batteries.