Merge Layers and Flatten Image
Photoshop CS6 Tutorial
How to Merge Multiple Layers or Flatten Image in Photoshop
Layers in Photoshop allow you to enhance, restore and build images in unbelievable ways. One way to look at layers is that each one in a clear acetate layer with or without additional content.
Often times you'll want to combine multiple layers into a single layer or take a whole stack of layers and flatten the image. The main reason you may want to flatten an image is to simplify the file for printing.
Open an image in Photoshop (File > Open) then open the Layers panel (Window > Layers).
Click the Text tool (t key) and add some text to the image.
Use the Shape tool to add a Rectangular Shape layer to the document. The Layers panel should have three layers; the image, your text and a shape layer.
Turn off the visibility for the image layer, then select the Shape layer.
To combine the two visible layers, choose Merge Visible (Shift/Cmd/e (PC - Shift/Ctrl/e)) from the Layers panel pull-down menu.
To flatten the image to a single Background layer, turn the visibility back on for the image layer and choose Flatten Image from the Layers panel pull-down menu.