Many laptops still come with more traditional style batteries that are user replaceable and only come with one year warranties. These batteries usually drop significantly in how long they can power a laptop after that first year. These batteries are usually found on the bottom of a laptop and can be removed by pressing some switches to unlock the battery from the computer. Replacement batteries usually run between $30 and $150.
Most newer/thinner laptops, tablets, and convertible tablets take advantage of newer battery technology. These batteries usually last significantly longer than traditional style batteries, have longer warranties (~3 years), but are not user replaceable. If this type of battery needs to be replaced, the computer will need to be sent or taken to a computer repair center to prevent voiding any warranties. Replacement batteries are also much more expensive than traditional style batteries coming in at around $250.
Combined Battery Technology
Some companies, Lenovo for example, have combined both traditional and sealed battery technology in their laptops/tablets. A laptop could have both a sealed battery that lasts about three years and a user-replaceable traditional battery that will only last a year or so. This can provide some benefits such as being able to swap out batteries while the computer is still running.
Battery life can be advertised in many different ways: hours of use, hours of standby time, watt-hours (Wh or Whr), or ampere hours (Ah). You can also get an idea of battery life by how many cells the battery has. In general, the more cells a battery has, the longer the battery will last.