SIM900 is older type (now obsolete), predecessor SIM800.
SIM800C has bluetooth on it,
SIM800L has FM on it.
SIM800F is pin compatible to SIM900 module.
SIM868 is dual sim version with GNSS and small form factor.
SIM800 use same AT commands as SIM900 + few new.
SIM900: http://simcomm2m.com/En/module/detail.aspx?id=71
SIM800: http://simcomm2m.com/En/module/detail.aspx?id=138
SIM800L: http://www.simcomm2m.com/phone/module/detail.aspx?id=13
SIM800C: http://simcomm2m.com/En/module/detail.aspx?id=74
SIM800F: http://simcomm2m.com/En/module/detail.aspx?id=130
SIM868: http://simcomm2m.com/En/module/detail.aspx?id=145
SIMCOM datasheets and firmware: http://simcom.ee/documents/