From the World Heritage inscription:
This tomb, built in 1570, is of particular cultural significance as it was the first garden-tomb on the Indian subcontinent. It inspired several major architectural innovations, culminating in the construction of the Taj Mahal.
Humayun’s Tomb is the burial site of the second Mughal Emperor, Humayun. It was commissioned by his son Akbar, the third Mughal Emperor, in 1569.
The tomb is a classic example of Muslim Mughal architecture, with many of the features of the tomb appear in later constructions such as the Taj Mahal.
The detail which can be seen in the structure is impressive and costly. All of the windows are covered with a jali, which is a carved, geometric screen, cut from a single piece of stone.
Surrounding the tomb building is a charbagh, or a four-part garden, which was the Persian style. There are also several smaller tombs which are the resting places of other servants and nobles.
If possible, I’d recommend visiting Humayun’s Tomb before visiting the Taj Mahal, as it will give you a sense of development of the architecture which lead to the creation of the Taj.