A document is a stack of layers. The Layers palette (Window ▸ Layers, Ctrl+4) lists them from top to bottom — an eye to hide or show each one, a thumbnail, the mask thumbnail when the layer has one, and the name — and its header sets the blend mode, the Opacity and the Fill of the selected layer. Every command below is in the Layer menu; most are also in the palette's own menu.
Layer ▸ New Adjustment Layer ▸ adds a layer that applies Levels, Curves, Hue/Saturation, HSL Curves, Color Wheels, Invert, Brightness/Contrast, Color Balance, Selective Color or Color Lookup to everything below it without changing any pixel; Layer Content Options… reopens its settings.
Layer ▸ Layer Mask ▸ adds a pixel mask that Reveals All, Hides All, Reveals Selection or Hides Selection; later you can Delete it, Apply it to the pixels, or Disable and Enable it. Layer ▸ Vector Mask ▸ does the same with a path (Reveal All, Hide All, Current Path, Delete, Disable). Painting on a mask thumbnail edits the mask.
Group Layers and Ungroup Layers (Ctrl+Shift+G) fold layers into a group. Create Clipping Mask (Ctrl+Alt+G) clips the active layer to the shape of the layer below; the same row reads Release Clipping Mask afterwards.
A file placed with File ▸ Place Image… as an SVG, PDF, AI, EPS or TIFF — or SVG artwork pasted from the clipboard — is a smart object: the layer keeps the placed file and is rendered from it. Layer ▸ Smart Objects ▸ has Edit Contents (an SVG opens in Vector Designer for Windows), Replace Contents…, Export Contents… and Show in Explorer; Layer ▸ Rasterize ▸ Smart Object turns it into ordinary pixels.
Every one of these commands is a step in the History palette (Ctrl+Alt+9) and can be undone with Ctrl+Z.