Raster Graphic:
A raster-graphics image, digital image, or bitmap, consists of a data file or of a data structure representing a generally rectangular grid of pixels, or points of colour, viewable via a computer monitor, paper, or other display medium.

A bitmap or a raster image corresponds bit-for-bit with an image displayed on a screen, generally in the same format used for storage in the display's video memory, or maybe as a device-independent bitmap. Computer scientists characterise a bitmap by the width and height of the image in pixels and by the number of bits per pixel, which determines the number of colours it can represent.

The word "raster" has its origins in Latin rastrum (a rake) and Latin radere (to scrape), and recalls metaphorically the systematic sampling of a grid-pattern of individual pixel-spaces with a view to representing an overall image.

Source: Wikipedia
http://en.wikipedia.org/wiki/Raster_graphics

In Photoshop, explore the following tools:
Rectangular (and Circular) Marquee Tool - used to select a section of an image and paste it in a different layer or another image file.
Move Tool - used to move different elements, often layers
Lasso Tool - used to select specific areas. Quite useful for advance collage techniques
Brush Tool - used to draw lines of different thickness. Sometimes desired for specific effects.
Rubber Stamp Tool (also known as Cloning Tool) used to clone from an area to assimilate areas (an advanced form of erasing material and supplanting with another element)
Eraser Tool - used to erase an area, often times with a specific color
Crop Tool - used to select a specific area and crop it

Image size is important. The higher the number, the more detail the image will have, the smaller the number the less detail. This is contingent upon the width and the height allocated to the image.

Layers - used to separate elements and have greater ability for manipulation
to use layers, select an area with any select tool described above and paste it. A new layer will appear. You can also create a new layer by clicking the "create new layer button" at the bottom left of the layer palette.
Color - can be accessed via the color palette at the bottom of the tool palette or in the color palette at the top menu, select: Window> Color

Image Mode - Important to keep in mind. For all printed material you should set your image to CMYK. For material designed for screen, you should choose RGB. Go to the top menu and choose Image Mode> RGB or CMYK (your choice)

To adjust the contrast and color balance of your image you have several options. Go to Image> Adjustments then choose from any of the choices. These features will be looked at in more detail in the future.

History Menu - keeps track of all your history for each session. Once you save the file and reopen it. It will lose your history. The history is useful if you realize that you did something wrong and need to go back several steps. To do this, bring forward the History menu by choosing from the top: Window. History then choose the desired action.