The Taj Mahal and the Agra Fort in the city and Fatehpur Sikri nearby. There are also many other buildings and tombs from Agra's days of glory as the capital of the Mughal Empire.
The city has little else to recommend it. Pollution, especially smog and litter is rampant and travellers are pestered by swarms of touts and hawkers at every monument, mosque, temple or palace. That said, the sites are some of the wonders of the world and no trip to India is complete without at least one visit to the Taj.
While Agra's heyday was as the capital of the Mughal empire between 1526 and 1658, the city was founded much earlier. The earliest reference to Agra is in the ancient epic, the Mahabharata, while Ptolemy was the first person to call it by its modern name. The recorded history of Agra begins around the 11th century, and over the next 500 years, the city changed hands between various kings, both Hindu and Muslim.
In 1506, Sultan Sikandar Lodi, the ruler of the Delhi Sultanate, moved his capital from Delhi to Agra. His son Ibrahim Lodi was the last ruler of the Lodi dynasty, as he was defeated in 1526 by Babur, the first Mughal ruler, in the battle of Panipat. Agra fell too, and became the capital of the Mughals, whose rule over Agra was uninterrupted except for a brief period between 1540 and 1556. In 1540, Sher Shah Shuri overthrew Humayun became the ruler of much of North India, including Agra. After Sher Shah Suri's death his descendants proved unequal to the task of ruling the kingdom, and Hemu, a Hindu general of Suri became the effective ruler who would later crown himself King Hemachandra Vikramaditya just as the kingdom was facing an assault from the reinvigorated Mughals. In 1556, Hemu would be defeated and killed in the second battle of Panipat, and the Mughals regained Agra.
Mughals were great builders. Babur built the Aram Bagh (garden of relaxation) modeled after the garden of paradise, where he was eventually buried after his death. His grandson Akbar refurbished the Agra fort and built the Fatehpur Sikri, an entire city just on the outskirts of Agra. He also renamed Agra after himself, and the city was known as Akbarabad while it was in Mughal hands. Akbar's grandson Shah Jehan would give Agra its most famous monument, the Taj Mahal, which is the mausoleum of his beloved wife, Mumtaz Mahal. The Taj is constructed in white marble. It took 20 years to construct, and is now universally known as a monument to love. Legend has it that Shah Jehan wanted a replica of the Taj constructed in black marble that would be his final resting place. There is no actual support for this theory, but even if it were true, it would have been unlikely to be eventuated. His son Aurangzeb was austere and pious, and had no time or inclination for the ostentation of his forefathers, preferring to spend his money on wars in South India. In any case, even during Shah Jehan's reign, which was the period when the Mughal empire was at its height, the construction of the Taj put a strain on the resources of the empire and caused a min-famine around Agra. Shah Jehan was eventually buried in the white Taj, next to his beloved Begum.
Shah Jehan, in addition to giving Agra its greatest claim to fame, was also responsible for beginning its decline, as decided to shift his capital to Shahjehanabad, which we now know as Old Delhi, in 1658. Though Aurangzeb ordered a move back, this too was short lived, as he moved his headquarters down south to Aurangabad to be focus on his wars. Agra declined, and so did the Mughal Empire. The city was eventually captured by the Marathas, who renamed it back to Agra. In 1803, it came under the British, who situated the Agra Presidency there, and when India gained independence, the city was incorporated into the state of Uttar Pradesh, and did not gain even the limited honour of being the state's capital, that distinction going to Lucknow, further east. It is now a tourist town, known for the Taj and a couple of other monuments.
Anyone interested in reading a novel based on the remarkable story behind the Taj Mahal's creation should consider Beneath a Marble Sky by John Shors. Beneath a Marble Sky is an international bestseller, has won multiple awards, and is being made into a movie by Hollywood. Other book (historical fiction) is The Taj by Colin De Silva.