!! If you search google you will see lots of information and advice that you should use professional software to safely resize partitions. Don't know how true this is and perhaps others here can give further comment for you. There is a Partition Resizer that I believe is free at click here but please heed above unless you get the nod from others on the forum.
in the accepted sense of the word chaztait, they are storage devices and contain files. I suspect what you mean is that you have one hard drive that's divided into two partitions. Windows refers to these partitions as drives, and labels them accordingly.
By default the operating system (and all other software) will install itself in the 'C' drive, and Windows will want you to store all your data files there as well - in the 'My documents' folder. After a while the C drive starts to fill up, and you're left with a 'D' drive that's empty. It's tempting to increase the size of the C partition and decrease the D drive, and that's why JaßîsFaß posted the link to PartitionMagic - it does the job for you.