There have been a lot of improvements on the standard battery in the last couple of decades and most now have additional features; some of these are quite important while others are just useful to have. As a general rule, the more additional features, the higher the price of the battery pack.
Protection against heat
Some batteries monitor their own temperature while they are being used or charged. If they get too hot they will change their usage to allow the cells to cool or indicate the problem by lighting an LED which allows the user to take steps to reduce the temperature. This helps maintain the service life of the battery as heat can damage it irreparably.
Protection against current
Some batteries will cut out during use if the current gets too high. A high current can also damage a battery irreparably. The battery may be designed to reset itself and continue within a few moments.
There are multiple cells in a battery and just one getting damaged can reduce the efficiency of the whole pack. Monitoring each cell individually is a way of ensuring they are all working to their maximum efficiency and identifying damaged cells.
Protection against overuse
Batteries that are designed to protect themselves against overuse cut the power to the cordless power tool when the amount of energy they can provide drops below a certain point. The power tool will just stop instead of running slower and slower as it would if the battery did not have a cut-off feature.
Charge level indicator
Some battery packs have a button you can press to show how much charge is left. They usually show green lights for fuller batteries and red lights for batteries that need to be charged.