Im new to computers and I'm wandering what 512 MB ddr means and what's better?

512 MB ddr = five hundred and twelve megabytes double data rate.

Have a look here for a fuller description of ddr RAM

Curlyben is 100% correct. This 512 MB DDR refers to the amount and type of RAM in your computer.

You have 512 MB of RAM which the computer can access on both sides of the CPU clock cycle. This means that the computer can read and write to and from the RAM at a speed twice the speed of traditional RAM.

512 MB is considered to be standard for personal computers on the market today, leaning towards "low-end" machines. If you use your computer for browsing the Internet, Instant Messaging, sending e-mails and basic word processing then 512 MB is suffecient. If you work with graphic-intensive programs such as graphic design, web page design or you have multiple peripherals connected to your computer in use at once or use a lot of programs all at once, you may want to consider an upgrade to 1 GB of RAM, which would be double your RAM (1 GB = 1,024 MB)