thanks to leave reply. I’m glad to help you out with your problem. I hope this might be useful.

If you have Background layer in your image using all Merge Layer, Merge Visible and flatten Image command to merge ALL layer will have same effects.

If you finish you work and want to distribute it, especially in file format other than photoshop file (.psd). You have to flatten it first because most of image file format (jpg, gif) not support layer. Flatten Image will discard all invisible layer.

Merge and Merge Visible are similar, they both use when you want to combine two or more layers, but different in the way you select layers to merge.

Merge Visible will merge all visible layer in the image. You can select layer to merge by making it visible, and all invisible layer will still there.

Merge Layer will merge select layer down to next below it. Or merge multiple select layer (for PhotoshopCS2).

In most case merge layer and merge visible will leave more than one layer. For image that don’t have Background merge ALL layer will result in one layer left. (still be layer that not support in other image file)

More over, Merge Layer command will turn to “merge clipping mask” if you select layer that have layer clipping. This will merge all layer in group. And if you select layer set this command will turn to “Merge Layer Set” and can merge all layer in its set to one layer or if selected layer have link to other layer command will turn to “Merge links” that can merge all linked layers.

By the way I already post article about Merge Layer, Merge Visible and Flatten Image in more detail here

http://www.freetimefoto.com/photoshop/merge_layer.html