RFID is Radio Frequency IDentification. It is an automatic 'radio tagging' identification technology whereby digital data encoded in an RFID tag or “smart label” is captured by a reader using radio waves.

RFID uses electromagnetic fields to automatically identify and track tags attached to objects. The tags contain electronically stored information. 
Put simply, RFID is similar to barcode technology but uses radio waves to capture data from tags (or RFID labels), rather than optically scanning the bar codes on a label. An RFID scanner does not require the tag or label to be seen to be able to read its stored data, but of course a label may be printed with a barcode and other text. An RFID printer programs the RFiD chip in a label and also prints text and/or barcodes on the label.

RFID is similar to barcodes, except that:
RFID labels can be read without being visible. (Unlike barcodes, RFID does not require "line of site").
RFID labels can be written to as well as read. (Typically when printing the label, the tag is also programmed).

RFID Frequencies
Two different frequencies are available for smart labels - ISO 18000-3 High Frequency (HF or 13.56 MHz) or ISO 18000-6 Ultra High Frequency (UHF ISO or Gen2). Typically hardware suitable for use with one frequency will not work with the other.

RFID Label Printers

RFID label printers print a label with text and (optionally) barcodes. At the same time as printing, the RFID chip in each label is programmed. High Frequency (HF or 13.56 MHz) or Ultra High Frequency (UHF ISO or Gen2) tags can be supported, depending upon model.

RFID printer

Mobile RFID Readers

Mobile handheld readers allow you to read tags on the go, and have an integrated antenna and display. Some existing mobile computers can have RFID capabilities added..

zebra handheld rfid reader

USB RFID Readers

Tethered USB readers have an integrated antenna and are typically connected to a PC.

zebra handheld rfid reader

Fixed RFID readers 

Fixed readers have a number of antenna ports (1-16) and also have network connection (Ethernet, possibly WiFi, possibly USB or Serial).
Fixed RFID readers require antenna . 

fixed rfid reader

Integrated RFID readers

Integrated RFID readers are fixed readers with built-in antenna..

thingmagig rfid reader