I just wanted to download an MX960 image and noticed the download center offers 2 types of images : Junos and Junos SR, what are the differences between the two ? in which case do you need to use Junos and in which case to use the Junos SR ?
SR is for Service Release. These are versions posted that are intermediate releases available between normal releases. Typically you only use them if you need fixes for particular bugs that have not rolled up to the main release yet.