There are a lot of frequencies in the RFID space, the main ones that pertain to access control are 125 kHz (LF), 13.56 MHz (HF), 900MHz (UHF) and Bluetooth.  125 kHz is commonly used; it is a low-frequency, it will correlate with short range and low security/ encryption.  13.56MHz is high-frequency, it will correlate with mid range and better security / encryption. 900MHz is ultrahigh-frequency and will correlate with long range and high security / encryption. 

All of the following frequencies (other than Bluetooth) rely on passive communication, the reader is constantly looking for a device that is within that frequency range. A few readers that correlate with frequencies are below  - 

125kHz - Prox readers

13.56MHz - Iclass readers, MiFare readers

900MHz - Needap, SEOs readers

Finally Bluetooth is a different communication altogether, it is not a passive frequency. All mobile credentials will communicate with this or NFC. This is the most secure format for a card / reader. It has a software layer encryption. Bluetooth can work across any of the different bit lengths.