It was more so in the past, but now they're getting a little more defined as two different apps. Elements is still very much like full Photoshop, but has more easy solution features for beginners getting more focus.
Basically, most tools found in both have more features and settings in Photoshop, and file formats like CMYK are missing in Elements, as well as channels and paths
in the layers panel.
I've been using Elements the last 5 years or so, but just got the CS4 Production Premium, so I'm back to proper Photoshop for the first time since version 5.
Unless you need the higher end features and pro file formats, You can do more with Elements than you'd think for the difference in price.