Maintenance agreements are service level agreements between me and you or your organization, that describe the support services provided for a specific part in your IT infrastructure.

IT infrastructures are split into several parts

End User Support, provide support services for the end user computer and any corporate assigned asset, such as mobile devices and printers.
Also it covers and ensures the proper operation of all the the related applications, such as Microsoft Office, Emailing, Anti-virus, backup, file sharing and any other business applications you have on your premises.

Servers and Storages, provide support for physical and virtual servers.
This includes file servers, application servers, domain servers, network policy servers, etc ...
Maintaining servers includes constant security patches and upgrades, backups, monitoring and logging.

Network Infrastructure, provide support services for internal Wired and Wireless network devices, like wireless access points, switches, routers and firewalls.
This part will cover proper implementation of the network based on international standards and best practices, constant patches and software upgrades to the related devices, tailored Quality Of Service ( if supported by the hardware on your premises ) to insure accurate and stable data flow for business related applications.
A proper network setup is key to a fast, secure, scalable and healthy environment, as it is the connection between the End User and the Servers.

Billing for such services depends on many factors, how many users, servers, how big is your environment, how much support you require, etc ...
All of which will be discussed after a meeting, where a site survey will be conducted to collect all the details required.