Unfortunately there is no way to explain this in layman's terms jonas. I would recommend that if you are interested in recording techniques you get a subscription to electronic musician (more for amateur/home studio) and MIX (pro).
This is as simple as I can put it.
Small Diameter Mic: Not very warm sounding but can take high SPLs w/o breaking up. Normally large diaphram mics are used for studio recorded vocals because they give more "body" and are more sensitive to microdynamic inflections.
Stereo Mic: A mic with two capsules one on top of the other. These can be rotatedup to 180 degrees of each other.
Cardiod pattern: As the name would suggest Doc, this is a heart shaped pick-up pattern.
Here as the source of the sound moves off-axis there is a drop off in spl. In this case -6dB at the 6 o'clock position.
With a stereo mic in cardiod this is accompanied by phase shifts.
example.
Play only one speaker and face it directly. Slowly turn counter clockwise and note how the sound changes in terms of tone and image specificity.
Your post suggests that the two Abba singers were positioned at either 12 and 6 o'clock where one would sound like the lead while one would sound farther away and diffuse or 3 and 9 o'clock where when the L and R signals are panned to center on the mix will make them sound like the harmony from their voices was coming from only one mouth but in a strange way sound like the polarity is inverted. This I believe is what you refer to as the "Abba" sound.
FET this refers to the phantom power (48V) source of the microphones.
Since mics are essentially speakers in reverse think of this as the FET or MOSFET poweramp with the attendant sonic signature being sufficiently fast, warm and smooth but lacking in inner detail and usually a tad dark and with a touch of electronic grunge.
All these are still in use today especially for movies and film.