What packages and repositories are maintained (supported)?
Not all packages in the main repository are maintained (supported) equally in an LTS release: Server packages are maintained for 5 years, and all packages that are part of the the seeds "server-ship" or "supported-common" are supported for five years.
For regular releases, all the packages in main are maintained for 18 months.
Packages in all other repositories (universe, multiverse, etc) are not officially maintained but may be getting some support from the community or an ISV. Be aware that apt does not check if a package is maintained (supported) or not, you have to do that on your own.
For more info:
To check out the seed, see germinate's output at: http://people.ubuntu.com/~ubuntu-arc...minate-output/
The ubuntu-maintenance-check script allows to check the maintenance cycle of each package installed on a server.
More details on how seeds are managed in Ubuntu is available at https://wiki.ubuntu.com/SeedManagement
What happens if I install packages that are not supported? Are there security patches for five years for those packages also?
If you install packages that are not supported, you are on your own. No security update is guaranteed to be provided for unsupported packages or bug fixes.