On April 17, I attended a very illuminating talk by the Venerable Bhikkhu Bodhi in Cupertino, CA, about 9 minutes walk away from my house.  The Venerable is, in my knowledge, the foremost scholar of Buddhism in America today.
The audio of the Venerable’s talk is available here.
(The first few minutes of the audio is really bad, but gets a lot better pretty soon).
The talk is about the Nikayas, the collection of the original teachings of the historical Buddha.  Here are my notes:
The collection of the original teachings by the Buddha are known as the Nikayas in the Pali tradition, and as the Agamas (阿含经) in the Chinese tradition.
Short history of early Buddhism: Many years after the passing away of the Buddha, Buddhism split into different sects.  For details, see: http://en.wikipedia.org/wiki/Pre-sectarian_Buddhism
Most of the records of the teachings of early Buddhism were lost with the demise of Buddhism in India.  Fortunately, 2 lines of transmission occurred and survived.
The 1st was the Pali Nikayas, which survived in Sri Lanka via the Theravada school of Buddhism, which thrived there.  The 2nd was the Agamas, which passed from northern India to China.
Each of these contains 4 collections, with names carrying largely identical meanings in Pali and Chinese:
1. Digha Nikaya (长阿含经), collection of long discourses.
2. Majjhima Nikaya (中阿含经), collection of middle-length discourses.
3. Samyutta Nikaya (杂阿含经), collection of themed discourses.
4. Anguttara Nikaya (增一阿含经), collection of “increase by one” discourses.
The main difference between the Nikayas and Agamas:  All the Nikayas are from the same school of Buddhism, which is the Theravada school.  In contrast, the Chinese translators chose the collection from a different school for each Agama.  For example, they chose the Dharmaguptaka version of the long discourses, the Sarvastivada version of the middle-length discourses, and so on.
There are some differences between the nikaya and agama collections.  The order of the discourses are not always identical.  Also, some discourses that are assigned to one collection in the nikayas are assigned to another in the agamas => assignments are not identical.
Implication: Very likely, the collections were not finalized at the First Council.  Venerable’s conjecture: Likely that the system of 4 collections are devised at that time, some important discourses assigned, but later over time, more and more discourses got added and different schools assigned them to different collections.
Interesting question:  What is the purpose of this scheme (of 4 collections)?  Is there a governing purpose?
Answer:  There is probably a governing purpose deeper than just the superficially obvious assignment by length and numbers.  Likely purpose:
– Majjhima Nikaya (中阿含经), collection of middle-length discourses.
This collection deals with various aspects of monastic training.  Its general governing aim is to bring together a large number of discourses that introduce newer monks to Buddhist teachings and practices.  Eg, the Four Foundations of Mindfulness, which is the basic scheme of Buddhist meditation practice.  Eg, discourses on defilements of the mind, purification of mind, right view, karma, Four Noble Truths, dependent origination, etc.
– Samyutta Nikaya (杂阿含经), collection of themed discourses.
Organized around a number of important doctrinal categories.  Eg, Dependent Origination, Five Aggregates, Twelve Sense Bases, the Path Factors, the Noble Truths, etc.  Collection deals with main philosophical ideas of early Buddhism.  The likely purpose of this collection is to enable monks specializing in deep investigation of Dharma to have all the materials in 1 place.  Also includes the discourses dealing with topics needed for development of prajna (wisdom).  This collection is most useful for meditators already well-established in their own practice and want to cultivate further to develop insight and wisdom.
– Anguttara Nikaya (增一阿含经), collection of “increase by one” discourses.
Collection organized by numbers.  A whole “book of ones” where Buddha said, “Monks, there is one XXX” (eg, one thing that is intractable, and that is the untamed mind).  Followed by “book of twos”, and so on.  Collection also contains a lot of advice and instruction for lay people.  Likely a convenient scheme to help senior monks find materials for giving lectures to junior monks and to lay people.
– Digha Nikaya (长阿含经), collection of long discourses.
Contains discourses countering positions held by rival teachings and teachers (during the time of the Buddha), and also discourses that glorify the Buddha.  Eg, a discourses that argues against 60+ non-Buddhist positions, the discourse on the death of the Buddha, a discourses where the king of gods asked the Buddha for his teachings (which he subsequently gave) etc.  Likely purpose of this collection is to help propagate Buddhism in an environment that would not be favourable to the Buddha’s teachings.