A feathered brush means that the edges will slowly fade into nothing thus giving a soft edge. Uhm, quite hard to explain here, but basically it makes the transition smooth, so whatever edit you're making won't have a sharp edge. You can also have feathered masking, feathered eraser etc.
one more novice question: why do we need different layers in PS? and what is the difference between a 'normal' layer and an adjustment layer? why can't i just do all my PS work on the original photo itself? :embrass:
Because if you realise you made a mistake or didn't like the changes, you can't get it back. Yes, there's the History column where you can easily undo the changes, but once you go past the max number of steps, you can't retrieve it. Trust me, it can go rather fast when you use a brush. You just keep clicking and clicking before you realised that you've gone past the max number.
Thus, it's always a better idea to duplicate your layer and make the adjustments on the adj layers. It's like keeping the negative untouched.
Just to add on, a "normal" layer is a layer with an image on it, be it the original "Background" or a duplicate of it or whatever, and an adjustment layer is one which stores a particular adjustment setting, be it brightness/contrast, levels, or some other.
By using an adjustment layer and not applying the adjustment onto the image itself, we have the advantage of, as was mentioned above, being able to remove a particular adjustment even after we have saved the file, closed it, and then reopen it anytime down the road. Or we can even tweek the adjustment instead of redoing the adjustment from scratch by double clicking on the adjustment layer in question (on the logo in the layers window).
If you wanna copy another picture in and you want feathered edges.
Click on the Mask Tool (the one with the dotted line) and choose the amount of pixels you want to feather by (type the figure inside the Feather: 0 px column along the top). Drag and mask the area you want to copy over (by not to the edges) and drag and drop to the collage.
If the picture in the collage is already in the collage and you want the edges to be feathered, you can do the same provided it's on a separate layer on its own. You can also use a large feathered eraser to delete the edges.