Ayutthaya (Ayudhya) was once the capital of the kingdom of Ayutthaya, also known as the kingdom of Siam. Siam went through several incarnations with the Ayutthaya period lasting from the mid 1300s until the mid 1700s.
This period is frequently regarded as something of a golden age for the Siamese people. Prior to its violent destruction the Ayutthaya kingdom grew progressively in both size and power. By the year 1700 the city of Ayutthaya had a population of approximately 1 million people making it one of the largest cities in the world at the time.
The kingdom was friendly towards foreign traders. Their policy of allowing the movement of traders and good through their territory saw the kingdom become a major trading hub for traders from as far away as China and France. Presumably this also helped bring in the money that would fund the construction of such a large city.
As it grew in size the kingdom of Ayutthaya had frequent conflicts with Malays to the south, Burmese to the west and Khmers to the east. Frequently these conflicts went in Ayutthaya's favour enabling them to add more territory to the kingdom. The many conflicts with the Burmese were especially frequent and bitter.
In 1767 a Burmese army was able to reach and then sack the capital carrying off anything of value and destroying everything else. This is the origin of the ruins that we see today. Their many conflicts remain the source of considerable ill will and distrust between modern day Thais and Burmese, particularly from the Thai side.
Today Ayutthaya has become a major tourist draw. While it may not have the mystical appeal of Angkor Wat it is accessible from Bangkok and well worth spending a few hours wandering around. You can get there by train or there are numerous companies running day tours from Bangkok.