The northeast holds a rather distinct place for most Thais. Its something akin to the nation's heartland. Generally poorer than the rest of the country, this is where much of the nation's rice and other staple crops are grown. The region has a specific name, which is isan (the "I" is pronounced with a long "EE" sound.) Isaan also has a specific regional cuisine which is quite distinct from the central Thai food you will find in most western Thai restaurants.
Isan also has a rich history, with evidence of occupation going back at least 5,000 years. The region is near the heart of the old khmer kingdom, and the ruins of khmer Hindu and Buddhist temples are found throughout the area. In recent years, some rich fossil finds have also sparked keen interest, with a few "Jurassic Parks" to be found here and there.
Isan is probably the least touristed area of Thailand. Its an excellent place to spend time trying to see the "real" Thailand.