4. The Vampire Lord's complete set of abilities
As a Vampire Lord there are two combat modes available, Blood Magic and Melee. In Blood Magic mode you hover and can cast various Blood Magic spells, whereas in Melee mode the emphasis is on attacks with your claws.
New perks on the Vampire Lord skill tree are not unlocked by levelling up in the usual way, but by using the Drain Life Blood Magic spell or power attack bite, which is triggered by executing a power attack on an enemy with low health but only occurs randomly. Focussing on these two attacks will open up new perks much quicker.
There are other benefits to having access to the Vampire Lord form that might not be immediately obvious. While in this form and hovering above the ground, you can quickly and easily cross large areas of water. Additionally your carry weight limit does not apply while in Vampire Lord form, so even when over encumbered you can still move around at normal speed.
Should you decide you no longer want to be a Vampire Lord, you can be cured by completing the 'Rising at Dawn' quest, which is triggered by speaking to any barkeeper. Any progress made up the Vampire Lord perk tree is saved, and speaking to Serana later will give the option to return to being a vampire, carrying over all previously unlocked perks.