Icon of the eyedropper tool (found in tools palette) in Adobe Photoshop:

It is used to:

• read colour values from the image (seeing information of sampled colours in the info palette)
• sample colour values from the image to use that colour elsewhere
• making a colour a foreground colour when clicking with it on the colour
• making a colour a background colour when holding down the alt key and clicking on the colour.

Note that whenever you are using other tools (meaning that they are selected), such as pencil, brush, line, type or gradient tool, when you hold down the alt key, you switch to the eyedropper tool temporarily. You can then click on the image and select that colour as the foreground colour (or background colour).

The default sample size setting is Point Sample (you can set this in the options toolbar) which is used to pick a specific colour from one specific pixel.

IMPORTANT !!!

Whenever you want to read values from a particular area of the image (for example skin tones, colour of an object), you want to switch to setting other than Point Sample, for example 3 by 3 average or 5 by 5 averagee.

No colour that you see on the image is made only from one specific colour, but is a mosaic of different coloured pixels. For this reason you want to sample values of an average colour. By doing that you also avoid the risk of choosing a bad or noisy pixel accidentally.