Windows OneCare was a product from Microsoft designed to combine a number of common computer maintenance features---anitvirus, anti-spyware, automatic defragmenting, photo backups and in-depth support from Microsoft experts. It's no longer offered to new customers, but most of the included features are offered for free by Microsoft.

Discontinued

Microsoft stopped selling Windows OneCare in October 2009. Existing users can use the service until their current subscription expires, but new customers hoping to get the service cannot do so by purchasing a Windows Live OneCare license.

Microsoft Security Essentials

When Microsoft stopped offering the subscription-based OneCare, they began offering the free Microsoft Security Essentials, which offers protection from viruses, trojans and spyware. Users familiar with the interface of Windows OneCare will feel immediately comfortable with Security Essentials, which aims to be comprehensive and lightweight.

Other Features

Security was the main selling point of Windows OneCare, but not the only one. Some of the remaining Windows Vista and Windows 7 both, by default, offer automatic defragmenting. Microsoft does not offer online backup of photos, but Google's Picasa offers similar functionality.

OneCare Scan

As part of a promotional effort for OneCare Microsoft offered the free Windows Live OneCare Safety Scan. This browser-based tool allows Windows users without a OneCare subscription to use OneCare's malware scanner and to remove threats. As of October 2009, the page prominently links to Microsoft Security Essentials, but is still operational.

Source:

OneCare.Live.com: Windows Live OneCare service overview

AddictiveTips.com: Microsoft Security Essentials Review