Athlons are like a regular grade cpu made for the average home user and gamers.
Semprons are a cheaper grade cpu for people on a budget. Even though they are cheaper grade they are still good.
Opterons are high grade cpus designed for use in servers and workstations. They are higher grade because they need to be reliable for the kind of work they are meant to do. Because of there high grade Opterons are extremely good for overclocking so many overclockers go for them. Think of an Opteron as an Athlon made with higher quality materials and tested rigorously. If you're not gonna OC just go with an Athlon