Erected in the 17th century by Mughal Emperor Shah Jahan, the Taj Mahal is the greatest monument to love in human history. Constructed over a period of 21 years between 1632 and 1653, the mausoleum pays tribute to the emperor's second wife, Mumtaz Mahal, who died birthing one of their children. At a large human cost and an even larger financial one, the Taj is India's crown jewel and most recognizeable symbol. Just a couple miles up the River Yamuna lies the Agra Fort, the indestructible former home of a long line of Mughal Emperors, including Jahan.
And as far as Agra goes, that's really all she wrote.
A relatively destitute and nondescript city in a barren plain of India's Uttar Pradesh, Agra's chief industries are tourism and marble. The marble business is a largely commercial one, which is an important fact to remember.
Due to the scarcity of parking and abundance of hustlers in and around the Taj Mahal, it is adviseable to obtain a driver for your day in Agra, despite the fort and the Taj being the only two significant sights to see. What you should know prior to entering the vehicle is that the drivers will insist on taking you to shops that are "state-owned" and feature "fairly-priced" handicrafts made of real, native marble. What they won't tell you is that they network with specific shops in town to get a cut of whatever tourists spent, that nearly all the "marble" crafts sold in Agra are made of soapstone and that the prices are anything but fair. You will notice, should you enter into one of these shops, that none of the items have price tags. It's not surprising, as most shopping in India is done via the haggle system; however, it is because of this inherent flexibility that shopkeepers will start you at prices as high as $200 for a set of drink coasters and will generally refuse to go any lower, unlike shopkeepers in nearly all other parts of the country.

On the subject of the Taj Mahal (and also the Agra Fort, though hustlers are not in as great a number there), it is absolutely essential that you do not respond to anyone who claims to be a "government photographer," or, indeed, any non-tourist not uniformed in Indian Government attire. There are government photographers availabe both here and at the fort; however, these personnel will have official badges and generally be set up near the ticket booths. Also, do not pay to have your shoes checked outside the entrance to the complex. Only the mausoleum itself is a "no shoe" zone and this service is provided free before you walk up the steps to the tomb.
Once you get past these minor annoyances, the Taj Mahal and the Agra Fort will afford you views, memories, and beauty unlike any other in the world. Be prepared to be stunned!