Helpdesks are often the first tier of technical support, and are nearly always the primary customer-facing area of IT. A typical Helpdesk receives calls or self-service style requests for assistance and services its customers remotely rather than traveling and working face to face. Some Helpdesks primarily service employees of the company they are employed by, and others are intended to assist external customers.
The bread and butter of the Helpdesk is performing known-error fixes such as password resets and common application errors, as well as fulfilling requests for software installations and access to shared resources. If a user's needs exceed the scope of the Helpdesk, that issue is escalated to tier-2 and tier-3 support for further assistance.