There are many different types of encryption.
The basic idea is to to make something unreadable unless you know how to read it.

Something like a simple substitution cypher (exchange A &Z, B&Y, C&X, etc etc etc) can normally be broken by hand in a matter of a couple hours or less. Computers can do it in less then a second.

Using a 1-time pad (Basically a list of random characters that are "added" to your text in one of a few different ways) is completely impossible to break unless you reuse the 1-time pad or lose it.

Every other form of encryption falls between those two for how difficult it is to break. The basic idea is how long do you want to keep something safe? Using a 1024 bit RSA key would take a couple thousand years to crack with current technology, so you would be pretty safe using it.