Basically there are 3 types of calendars used widely in traditional times.
Nong Li
Nong Li calendar is also known as Farmers calendar, Hsia/Xia calendar, Luni-Solar. The above refers to a type of chinese calendar which uses both Moon calculation and Sun calculation to determine the year calendar.
Lunar calendar
This is also known as Yin calendar or Moon calendar. This uses 29.5…… days as a month for calculation.
Solar calendar
This is also known as the Yang, Sun, or Solar calendar. This uses the rotation of the sun around the earth. One full rotation is 1 year. The current one used is Gregorian calendar which is our standard 1st Jan 2009 etc.. and so on.
Why do we need to know and why convert between calendars?
There has always been conversion issues between Gregorian dates and Chinese Farmers Calendar. Why do we need to convert? Here is the important part. Almost all chinese predictions, calculations, even medications depend on Time / Date. The problem is alot of these methods are based on Farmers calendar and can be used on that. Therefore without a good conversion means, the results will be confusing, contradicting and wrong. This makes it hard to validate if methods are correct or if we can improve it.
A note of warning here, there are various chinese calculations and masters using the wrong calendar, or wrong conversion methods and thus results are ridiculous and misleading. please verify a couple of different calendars and see which is more appropriate.
References
http://baike.baidu.com/view/125449.htm
http://en.wikipedia.org/wiki/Chinese_calendar
http://ccal.chinesebay.com/ccal/index.html
http://www.timeanddate.com/calendar/moonphases.html