Merging Layers

Command (Mac) / Control (Win) + Shift + E will merge visible layers.
Command + Option (Mac) / Control + Alt (Win) + E creates a new layer and pastes a “flattened” version of the selected layers on it.
Command + Option (Mac) / Control + Alt (Win) + Shift + E creates a new layer and pasted a “flattened” version of all layers on it.