ARC4 is a symmetric encryption algorithm fast and easy to implement. Being symmetric does means that it uses the same function with the same key (varying from 40 to 2048 bits) both for cyphering and de-cyphering a block of data.
Is it the most secure or robust encryption algorithm around? Not really. But it provides a good amount of performance and you can take further steps in order to correct some of its flaws. So, continue reading if you are interested in having this one in your developer toolset implemented as a Class with separate methods to encrypt and decrypt a block of information (even if it uses the same function in both cases).