(From DMG pg 286, © Wizards of the Coast)
Whereas tieflings have fiendish blood in their veins, aasimar are the descendants of celestial beings. These folk generally appear as glorious humans with lustrous hair, flawless skin, and piercing eyes, and in their twenties (which is when they reach adulthood). Aasimar often attempt to pass as humans in order to right wrongs and defend goodness on the Material Plane without drawing undue attention to their celestial heritage. They strive to fit into society, although they usually rise to the top, becoming revered leaders and honorable heroes.
Ability Score Increase:
Your Wisdom score increases by 1, and your Charisma score increases by 2.
Aasimar mature a little slower than humans (around 20), and live about 150 years. The maximum age for a level one Aasimar is 60 yrs.
Due to their celestial heritage, aasimar are often good. However, some aasimar fall into evil, (such as Fallen Aasimar) rejecting their heritage.
Aasimar are built like well-proportioned humans. Your size is Medium.
Your base walking speed is 30 feet.
Thanks to your celestial heritage, you have superior vision in dark and dim conditions. You can see in dim light within 60 feet of you as if it were bright light, and in darkness as if it were dim light. You can’t discern color in darkness, only shades of gray.
You have resistance to necrotic damage and radiant damage.
You know the light cantrip. Once you reach 3rd level, you can cast the lesser restoration spell once with this trait, and you regain the ability to do so when you finish a long rest. Once you reach 5th level, you can cast the daylight spell once with this trait as a 3rd-level spell, and you regain the ability to do so when you finish a long rest. Charisma is your spellcasting ability for these spells.
You can speak, read, and write Common and Celestial.
An updated version of the Aasimar, including subclasses, is available in Volo's Guide to Monsters