18650 is the physical geometry: 18mm diameter, 65mm long, 0=round.
Any Li-ion 3.7V 18650 will do as a replacement (peak charge voltage4.2V) - but get a matched set of 5.
It's meaningless as far as you're concerned.
No. Replace the lot - you don't want a mis-matched string.
Buy replacements with tabs on. Soldering can damage the cell internally unless you're very skillful and swift.
Avoid Ultrafire and other *fire Chinese brands and their Chinese clones (often repackaged dead cells). They make fantastical claims of capacity, but you're lucky if you get 1000mAH, and never as much as 2000mAH (they often claim 4000 or 6000; no-one can build that).
If you can afford it, buy Panasonic NCR cells - about $12-$15/ea - which if genuine will produce about 3400mAH at a much better sustained voltage. Chalk and cheese against the Chinese cells.