Hello, Kris.
As Jaiboon pointed out, the temple that you visited probably describes itself as “Maha Nikaya” because it belongs to the Mahanikay sect. As far as I know, all the monks in the Thai tradition are divided into two sects: Mahanikay and Dhammayut (spelling in English varies). Below are a couple links that briefly explain the technical and historical difference between these two sects. In practice, however, the difference is not so clear.
Wikipedia Article
http://en.wikipedia.org/wiki/Maha_Nikaya
Wheel publication on Buddhism in Thailand
(see section called “Two Sects or Nikayas")
http://www.accesstoinsight.org/lib/authors/kusalasaya/wheel085.html
Within the sects, the monks are generally organized into what one can call lineages. For example, the monks who all consider Ajahn Chah their teacher, or their teacher’s teacher, are all considered to be in Ajahn Chah’s lineage. Ajahn Chah’s lineage is in the Mahanikay sect.
As I said, in practice the difference between the sects are not always clear. Some lineages might be closer in the way they practice to lineages that are in the other sect than they are to lineages in their own sect. Furthermore, monks from the different sects visit back and forth among each other’s monasteries learning from each other and sharing practice traditions. Therefore, the whole issue is mostly of importance in Thailand, among those who are closely involved with the politics of the monastic hierarchy. And many monks, both in Thailand and in the West, are not greatly interested in the politics or the hierarchy. For example, in the Thai monastery close to my house, they really try to play down the difference between the two sects. They hand out tapes and books from Ajahn Chah’s lineage even though they are a Dhammayut monastery.
Many might say that a more important distinction is the one between the forest monks and the city monks. On the whole, the forest monks emphasize meditation and are more strict about renunciation and simplicity. You’ll find forest monks in both Mahanikay and Dhammayut lineages. For example, Ajahn Pasanno and Ajahn Amaro are in a Mahanikay lineage, while the lineage of Ajahn Geoff (Thanissaro Bhikkhu) is Dhammayut. Yet they are both in the Thai forest tradition, so they are probably closer to each other in most ways than they are to monks in the city temples belonging to their own sects. In general, all Dhammayut monks and the forest monks in the Mahanikay lineages wear robes that are darker in color than the bright orange robes worn by Mahanikay city monks. But even that isn’t a sure-fire indication. If I recall correctly, the Ajahn Buddhadasa wore a bright orange robe even though his practice and teaching style was more like that of a forest monk.
There’s information about all this, and much more about the Thai forest tradition, in a book called “Forest Recollection” by Kamala Tiyavanich (ISBN 0-8248-1781-8)
Also, here’s a link to an essay by Ajahn Geoff that explains quite a bit about the Dhammayut sect (he calls it a movement) and the forest tradition (Kammatthana tradition) founded by Ajahn Mun. In the essay, Ajahn Geoff relates the Mahanikay sect to “Customary Buddhism” and the Dhammayut sect to “Reform Buddhism.” I believe that these terms are useful, but I don’t think they’re widely recognized.
http://www.accesstoinsight.org/lib/authors/thanissaro/customs.html
Note that, though Ajahn Mun was a Dhammayut monk, he had a great influence on Ajahn Chah.
So . . . it’s complicated. I hope I’ve spoken correctly. If anyone sees something incorrect in what I said, please let us all know.
Dorothea