In order to understand Photoshop, you have to familiarize the tools used by designers. The tools of Photoshop are divided into five parts: (1) Selection, Crop & Slice Tools, (2) Retouch & Paint Tools, (3) Drawing & Type Tools, (4) Annotation/Measurements, and (5) Color box.
Selection, Crop & Slice Tools
This includes marquee, move, lasso, magic wand, crop, and slice tool. Usually, marquee is used when you want to make a square/rectangular shape on your image. Move is understandable already(use to move things). Lasso is use to draw selections in any shape that you would like to create. Magic Wand is used to select a color range. Crop is self-explanatory. Use to crop your image. Slice is used to slice an image when you do slicing. Most commonly, I use this tool when I start to build a new website design.
Retouch & Paint Tools
This includes healing, brush, clone, history, eraser, fill/gradient, sharpen/blur, and dodge. Healing is use when repairing something. Brush is very useful and everyone love this because, this is the only tool that you don’t need to study. Clone is somewhat have the same capability to a healing tool. History brush works like a normal brush but the only difference is that it’s paints is from the original state of your image. Eraser is understandable as well. It works like a simple eraser. You can use it when you have made a mistake. Fill is used to fill color to a specific selection. Gradient is use when you are making a gradiation of colors. Sharpen is use when making putting some emphasis to your image corners. Blur can make things blurry. Dodge is used to lighten any area.
Drawing & Type Tools
This group of tools are used for vector based drawing and type. This includes path selection, horizontal type, rectangle and my favorite pen tool.
These are tools use for remarks and dimensions. This includes notes, eyedropper, hand, and zoom. Note is use if you want to make a small note to your image. Eyedropper is use when you like to change your foreground background color to whatever color you want. Hand is use to move the image. Zoom is understandable. Use to change the size of your image.
This contains boxes of colors. Foreground is the one in front and Background is the one at the back. You can click on any of them to bring up the color.