In dungeons and dragons, dragons come in many different flavors. Chromatic dragons (Red blue green etc) tend to be evilly aligned. Counter to that the metallic dragons (Silver, Gold, Bronze, etc) ten to be aligned with good. There are many dragons that don't fit these two archetypes including the gem dragons (ruby, amethyst) environmental dragons (smoke, mist)
Dragons come in all shapes and sizes from the tiny drakes up to the huge ancient variety. One thing all true dragons share is the ability to use a breath weapon. The flavor of this weapon changes from dragon to dragon for example the red breaths fire while the black spews acid. This attack always scales in power with how old the dragon is. Many dragons can also cast magic spells depending on their age they may even rival humanoid races in this area of expertise.
As a GM I tend not to use dragons very much, I feel that they should be very special encounters and memorable each time. In the last 13 or so years of running games I've use combat dragons maybe 5 or 6 times and each time the party remembers the encounter well. Dragons if played properly to their full extent can easily TPK (total party kill) a group.