These are by far the most common lithium ion batteries. With some (very rare) exceptions they are all rated 3.7v but actually operate between 3v and 4.2v – any lower or higher and they will be irreparably damaged. They are available in a variety of capacities, up to a maximum (currently) of 3,500mAh.
The 3,000mAh unprotected Li-Ion is the sweet-spot of price vs. performance, and I always recommend two sets of 3,000mAh cells rather than one set of protected 3,400mAh or 3,500mAh because it means you can charge one set while using the other, or carry a spare set for those priceless moments when you realise that you forgot to charge the ones in your torch after your last outing!
There's a lot of BS in the battery industry so be careful of Li-Ions that are very cheap or claim capacities of more than 3,500mAh – at best you'll waste your money, at worst they'll burn down your house. This independent website is a great resource on batteries and chargers – especially this "comparator".
Click on the pictures below to see details of my batteries and drop me a line to discuss discounts.