Jobs were introduced in PowerShell 2.0 and helped to solve a problem inherent in the command-line tools. In a nutshell, if you start a long running task, your prompt is unavailable until the task finishes. As an example of a long running task, think of this simple PowerShell command:
Get-ChildItem -Path c:\ -Recurse
It will take a while to fetch full directory list of your C: drive. If you run it as Job then the console will get the control back and you can capture the result later on.
PowerShell Jobs run in a new process. This has pros and cons which are related.