They are brush blend modes. To use you need to have the brush tool selected. Open a new layer and paint with any colour. Chose another colour and make the blend mode Behind. I bet you know what will happen now? ;-)
Clear works pretty much the same as the Eraser tool as far as I can tell. Might need to actually read the manual to get more info on that.
I am fully aware of the use of these blend modes. But my question was in relation regarding that these two new modes were not there previously. I wanted to find out why? Was it due to the files I downloaded? Was it virus related? Why all of a sudden I have two new blending modes when I never had them there before? This really baffles me.
Sometimes with the Brush Tool, those brush blending modes (Clear and Behind) are greyed out because of the image mode or layer selected (i.e a Background layer), but those brush modes: Clear and Behind have been in photoshop for many versions.