While laptops provide you with the ability to have the functionality of a desktop but in a more compact form there are other differences and one of the biggest is the ability to upgrade components in your laptop. There aren't as many that can be upgraded because of the design - more is integrated into the motherboard compared to desktops.
You can upgrade some components and knowing which can be upgraded at a later date may affect your decision to purchase a particular model. If you can save a bit of money now and later purchase upgrades as you need or can afford them.
1. BatteriesThe battery is a very important component in a laptop. Most new laptops have lithium polymer which is a more stable battery. Lithium polymer are not flammable and much lighter than lithium ion, so more power can be put into smaller batteries. The average operating time is between 3 and 6 hours. There are high capacity batteries available which can last for between 8 and 10 hours but you will pay more for those. The majority of laptop batteries will need to be replaced between 3 and 5 years depending on how often you have used the laptop on battery power only. NOTE - If you are able to purchase a 2nd battery and your laptop will allow you to install the second battery in a swappable bay port; then buy the second battery.
2. Hard Disk DriveThe hard disk drive (HDD) is where your operating system, applications and files will be stored. New laptop hard drives are designed to be shock resistant and have lower power consumption. Depending on the type and make of laptop, the HDD will either be 3.5" or 2.5" in size. Laptop HDD are quite thin compared to desktop hard drives and the capacities now available are a great improvement over past models. Program sizes and storage needs have really driven the hard drive manufacturers to keep pushing to get more space on smaller disks. NOTE - An 80GB SATA drive with 5400rpm or 7200rpm is an ideal size for the majority of laptop users.
3. RAM When your purchase your laptop an important factor to keep in mind is whether you will be able to upgrade the RAM at a later date. There are some models particularly in the ultra portable class which cannot have the RAM upgraded. At the time you purchased your laptop you may have only been able to afford the RAM that came standard with your model. At a later point you may find RAM at a lower price and that is the time to purchase more RAM. In your User Guide there will the specifications for the type of RAM you have and you have to be sure to match that in order for it to work properly in your laptop once installed.