Strays were rogue programs who lacked identity discs. During the rule of Clu, they would be rounded up by Guards and Sentries to be rectified. Sam Flynn was captured by Sentries when he entered the Tron system after it was found that he lacked a disc.
Strays often lost their memory in fits called glitches after being parted from their disc for too long. An example of a stray is Beck, who briefly lost his disc. During the period before he reobtained it, he constantly was in a state of instability. After reobtaining his disc, the glitches stopped as well as downloading his memory back.