What are all these categories? Why are they useful?

Every article which is posted on the website has been placed in appropriate categories.
For example, an observation report of a galaxy which contains images produced with a CCD camera will be found in the Galaxies category (found under Astronomical Objects and Phenomena > Deep Sky), as well as the Digital Images category (found under Observations > Imagery).

The categories work in such a way that as you drill ‘deeper’ in, they get more specific; thus, if you want to see all articles about any type of astronomical phenomena you can click on the Astronomical Objects and Phenomena category. This would bring up all the articles which were categorised as being in that category and those categories within it, e.g. everything categorised as just Astronomical Objects and Phenomena, plus everything in Deep Sky, everything in Solar System, etc.

Now let’s say that your interest is the planet Mars.
Although you will find articles about Mars if you click on Astronomical Objects and Phenomena, they will be mixed in with many other articles covering a multitude of topics.
If you were to drill down to Astronomical Objects and Phenomena > Solar System and click on the Planets category, you would see all articles relating to the planets in the Solar System. If you drill down a level further, you will find the Mars category. By clicking on this, you will see a list of articles (newest at the top) which are on the subject of (or contain significant references to) Mars.

If you look on the right of the homepage you will see a list of ‘top level’ categories.
You can click on a category to see the articles it contains, or click the [+] next to it to expand it, showing any sub-categories it may have. These may also have a [+] next to them.