They are awarded for different levels of actions. The Silver Star is a higher award than the Bronze Star. Both are given for high levels of performance not just for "doing your job". My dh received the Bronze Star for his actions in Afghanistan. I wasn't able to be there when he was presented the award but they did record the ceremony for me so I've got it on DVD. All the branches award both the Silver Star and the Bronze Star. I'm sure that the criteria are similar in all the services. If your co-workers has them mounted on his/her wall I'm sure that he/she's got the certificate that accompanied them that details why they were awarded the award.
Hope that helps.
Kristin