There are a number of ways to connect RFID readers to Raspberry Pi (and Arduino-style) devices, but it all boils down to interfaces.
The simplest interface for most of us is an RS-232 serial interface, and most of the pre-packaged readers use this, I think. The Pi doesn't have old-time PC serial and parallel ports, but a USB/RS-232 adapter is relatively inexpensive.
There are other types of interfaces as well, that can connect to the Pi's GPIO pins, but unless you do that sort of stuff for a living like me or you got a good deal on a surplus unit and want to save money, stick to the serial port option.
The actual output of RFID readers is usually just plain ASCII text, often nothing more than a verbatim copy of the data that is stored on the RFID tag or card.
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