Er refers to extended release. ER oxycodone is slowly released over 12 hours allowing a consistent flow of the painkiller to your tissues and more consistent pain relief.
Oxycontin is another name for oxycodone. CR refers to controlled release. This drug too is slowly released over time to provide a consistent does of the pain killer.
You have to be careful not to chew either of these. Because they were developed to be released over a long period of time they contain higher than normal levels of the painkiller. Thus if you chew them and take in all the pain killer at once you can actually overdose on them (!).