When I used to work at the Apple Store, we learned this really cool analogy for explaining computers and computer parts to our customers. Many people, myself included, don't really understand terms like RAM or Giga-hertz or dual-core processor. The problem is, when it comes time to purchase a computer, understanding what these things mean is key in order to get exactly what you need out of a computer! So without further ado, I'd like to introduce you to the kitchen anology.
PROCESSOR: Your processor speed is measured by GHz (giga-hertz) My iMac, for example, has a 2.66 GHz processor. In the kitchen anology, imagine that the processor is your chef - how skilled he is, how FAST he can work... that is your processor.
STORAGE CAPACITY: Your storage capacity is how many GB (or TB) of storage space your internal hard drive has. You usually start with a nice, round number like 500GB or 1TB or something like that. In the kitchen, the storage space is like your refridgerator and pantry space... how much physical space you have to store your ingredients. The smaller your fridge, the less ingredients your chef will be able to keep in the kitchen.
RAM: It's this tricky thing that is a bit more abstract. It is measured in GB also, but don't be confused with your storage capacity - RAM is your memory. In the kitchen anology, RAM is equivelant to the counter-top space your chef has. If your have a small counter, he can only do so many things at once, but if he has a lot of counter space, he can do many things at the same time. Think of it as your multi-tasking power.
They all affect each other in one way or another, and it's important to understand what your needs are in a computer. Certain programs, especially editing software like Lightroom and Photoshop, use a LOT of RAM. It's important to know these things before you purchase a computer because you want to know how much RAM you will be able to install. After doing some research, I found out that the base-model of the 21.5-inch iMac can only be upgraded to 8GB of RAM, while the upgraded 21.5-inch model (2.7GHz processor) has the ability to be upgraded to 16GB of RAM.
You can have a super fast processor, but if you don't have a lot of RAM, your "chef" can only get so much done. Consider how much RAM you can upgrade to and look into doing that if you find that you are having trouble running multiple applications, or even just one or two running at a time seems slow. Complex programs, like Lightroom and Photoshop, are like recipes with a lot of ingredients - they need a lot more counter space for the chef to prepare the meal. Programs like Safari and other small ones are like simple meals with a few ingredients, and you can probably run more of those at the same time without a problem.
The biggest factor for most people is budget - how can I get the most for my dollar? Well absolutely, without question, you can get more for your money with a desktop - it's virtually half the cost for almost double of everything else. But, if convenience is a factor, you can certainly find laptops that are specked out to the max, but you'll be paying more for it. Just consider what you will be using the machine for and how you can make the most of it. Nothing is more frustrating than a slow computer!
I'm certainly no computer expert, but this little analogy really helped me wrap my mind around these crazy terms that are thrown around in the computer world. I hope this was helpful for you, as well!
I hope you have a fabulous weekend!