explorer.exe is the Windows Program Manager or Windows Explorer. It manages the Windows Graphical Shell including the Start menu, taskbar, desktop, and File Manager. By removing this process the graphical interface for Windows will disappear.
Note: explorer.exe is also registered as a process which is the w32.Codered and the w32.mydoom.b@mm viruses. These viruses are distributed via the Internet through e-mail and comes in the form of an e-mail message, in the hopes that you open it’s hostile attachment. The worms have their own SMTP engine which means it gathers E-mails from your local computer and re-distributes itself. In worst cases these worms can allow attackers to access your computer, stealing passwords and personal data. It is a registered security risk and should be removed immediately.
If explorer.exe starts using more and more RAM while you do nothing, please post a Hijackthis log HERE