What is a web bug or web beacon?

Web bugs are pixels used for counting, that are placed in web pages or emails.

It is usually a transparent 1x1 pixel GIF image, but can in principle be any artwork, for example, advertising banner or logo of the visited page.

Purpose

Their purpose is to collect statistical information, for example:

How often a page is requested? As graphics, they allow a better distinction between man and machine than the usual log files on the server.
Which browser or operating system has been used? Further details regarding the user's computer equipment can also be detected with the use of Javascript. Of interest for web designers are e.g. screen sizes and installed plugins. Insights are often used for the optimization of the website.

Generally there is no direct threat implied by these pixels because they are graphics.

Like cookies, however, they can also be misused to invade the privacy of the visitor.

Embedded into emails they confirm the reading of the mail and thus the email address without further user interaction.
Like banners also counting pixel can be used to set cookies and to recognize a visitor across multiple pages that are in the control of the same operator.
Because that on one side every graphic could be used as Web bug, and on the other side 1x1-pixel are used for layout, web bugs are harder to suppress than cookies.

Use an email client, where you can turn off the automatic loading of images. Download images only for mail from known senders.

Avoid the display of graphics in SPAM or other advertising, to not verify your address and reading the mail.