Merging layers combines several selected Photoshop Creative Suite 5 layers into one layer. Flattening occurs when you reduce all Photoshop layers to one background layer. Layers can increase file size, thereby also tying up valuable processing resources. To keep down file size, you may choose to merge some layers or even flatten photoshop layers.

　　Merge layers

　　Merging layers is helpful when you no longer need every layer to be independent, such as when you have a separate shadow layer aligned to another layer and don’t plan to move it again or when you combine many layers to create a composite and want to consolidate it to one layer.

　　To merge layers (in a visual and easy way), follow these steps:

　　Turn on the visibility of only the layers you want merged.

　　Choose Merge Visible from the Layers panel menu.

　　That’s it. The entire image isn’t flattened, but the visible layers are now reduced to one layer.

　　To merge visible layers on a target (selected) layer that you create while keeping the visible layers independent, create a blank layer and select it. Then hold down Alt (Windows) or Option (Mac) when choosing Merge Visible from the panel menu.

　　Flatten layers

　　If you don’t have to flatten your image, don’t! Flattening an image reduces all layers to one background layer, which is necessary for certain file formats. After you flatten an image, you can’t take advantage of blending options or reposition layered items. If you absolutely must flatten layers, keep a copy of the original, unflattened document for additional edits later.

　　To flatten all layers in an image, choose Layer→Flatten Image or choose Flatten Image from the panel menu on the Layers panel.