Aha! A smart customer! This is a question that every customer needs to ask their web host, and we're glad you asked! Overselling is the bane of the VPS world. It can slow a brand-new high-performance server to a crawl and cause endless amounts of frustration. In some cases, it can even prevent you from using the full resources of your VPS – resources you're paying for!
A lot of web hosts – especially those using OpenVZ – oversell their VPSes. This means that if you order a VPS with 2 GB of RAM, you might not always have the full 2 GB available. It's the same with disk space: if you order a VPS with 100 GB of disk space, you might only be able to use 60 GB before running out of disk space – even though you should have another 40 GB to go.
That's why we never oversell our memory or disk space. When you purchase a VPS with 4 GB of RAM or 200 GB of disk space, that's exactly what you'll get. Because the CPU is shared among multiple hosts, we implement “fair share” scheduling so that no one VPS can consume all the CPU power. Some people consider this “overselling”. We disagree, but we're all about full disclosure, so here we are!