These can help here. Reference [1] is the best book I have ever seen in the field of RFID.
[1] Klaus Finkenzeller, RFID Handbook: Fundamentals and Applications in Contactless Smart Cards and Identification 2nd Edition, Wiley, 2003
[2] **broken link removed**
[3] http://www.uidcenter.org
Actually there is a lot more frequencies for RFid. i ll give you most popular and commercial
For Passive tags:
- 13.56 Mhz. (Mifare) nearfield(literature says up to 1 meter but i saw just 15 cm.
- 866, 915 Mhz. (UHF EPC Gloabal Gen1, Gen2, ans ISO 16000b, c) Frequency depends on region.
For Active:
Dod uses 433 Mhz.(Dual band. and some working of standardisation of commercial companies such as SAVI)
Working summary:
Reader transmits signal for passive tags, and tag backscatters the signal to reader.
this topic has a lot to say that it can not be fully explained with 2,3 sentences. so if you need more detail, contact me..