Dragonlords is the title for members of the forty noble families that vied to actually rule the Valyrian Freehold. They were called dragonlords because they could ride and control dragons. They controlled dragons with whips, horns and sorcery. They were strikingly [some say inhumanly]  beautiful.  It had long been the custom amongst the dragonlords of Valyria to wed brother to sister, to keep the bloodlines pure.
The Targaryens were a minor one of these families, far from the most powerful , but are the only one that survived the Doom of Valyria with dragons. Thus members of House Targaryen are known as dragonlords in Westeros, even after the death of all the dragons. House Belaerys were another family who could ride dragons.
Our histories speak of the dragonlords of dread Valyria and the devastation that they wrought upon the peoples of Old Ghis.”
- the Green Grace