Arrange, Merge Down, Merge Visible, and Flatten Image
Arrange: The arrange option lets you change the order of your layers. Some options will be grayed out depending on what layer you are working with. For example, if you are working with the front layer, you will not be able to choose bring to front because it’s already there. Bring to front will bring the selected layer up one layer, bring forward will bring it all the way to the front. Send back will send the layer back one, while send to back will put the layer at the very end of the document. Reverse will do just as it says; reverse the order of your selected layers.
Merge Down: Merging your layers will change your image from many layers to a flattened image with all of the changes appearing on one document, rather than each piece of the document being on a separate layer. Sometimes this will appear as merge layers in which case it will only merge the layers that you have selected together, rather than all of the layers in the palette.
Merge Visible: This will only merge the layers that can be seen in the document. If you’ve hidden any layers, they will not merge with the rest of the document.
Flatten Image: This last menu option will allow for you to merge every layer in your document into one image. It will take all of the changes in each layer and merge them into one photo reflecting each layer’s changes. This works well if you want to save your finished photo as a .jpg, .gif, or other extension. If there are still un-flattened layers, you will only be able to save the image as a .psd file.