Every tree has a range of features - leaves, flowers and other parts. Many of these features are shared with other trees. A few may be unique to that particular species of tree, but mostly it is a combination of features that separate one species from another.
One way to identify a tree would be to read descriptions of every known species and decide which one most closely resembles your specimen. But this would be impractical - and tedious.
Another way is to use a key. Keys divide potential candidates into smaller and smaller groups based on their features. They do this by asking a series of questions. For example, are the leaves lobed or not lobed? Each answer reduces the number of possible identities until you reach a single name.
The questions chosen and the order in which they are asked have nothing to do with biological or any other measure of importance. They are simply the questions that most conveniently separate different groups.
A key is simply a tool for identifying specimens. There are different types of key. This one consists of a series of steps leading to the name of the tree you are examining. At each step it asks you to decide which of two or more options most closely matches your specimen.
The questions tend to go from general to more specific as you progress through the key. The same question may also be repeated in different sections of the key. It is important to answer each of the questions asked.
Each answer leads you to the next step, where the process is repeated until you reach the name of a tree.
Selecting the name takes you to a factsheet for that tree. The factsheet gives additional information and images to help you confirm your identification.
All of the parts needed to make a complete identification - leaves, flowers and fruits - may not be present on the tree at the same time. Sometimes you might need to make several visits.