SCRs are generally for DC, and Triacs are better suited to AC.
They are rather crude, and noisy, but very tough, and very cheap.

An SCR will "lock-on" until Current is removed from it.
A Triac will also "lock-on", but, of course,
AC goes to zero 50 or 60 times a second, which then turns it back off.

FETs can be considered to be a sort of "variable-Resistor",
with a Diode thrown into the mix.
The Gate-Input basically looks like a small Capacitor to the driving Circuitry.
The Gate has almost infinite DC-Resistance, just like a Capacitor.
This Capacitance is usually not an issue when using a FET for a simple "On-Off" Switch,
but when making a FET Switch at very high-Frequencies,
the Gate-Capacitance can be a very significant design factor.

A FET acts like a simple Diode when "Off",
and acts like a very low-value Resistor when "On".
( Voltage may flow in either direction when "On" ).

"Back-to-Back" FETs will block AC when "Off",
but will pass AC when both are "On".
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SCR are mainly just diode bridges

Not a bridge in the usual electrical sense of that word.
SCR are basically 4-layer diodes which can be emulated by two transistors (below).
The have about a 1-diode forward drop when on, due to the junctions the current passes through.

i presume FETs are basically transistors

Yes, a MOSFET is a transistor (Metal Oxide Field Effect Transistor).
The MOSFET current does not travel through a junction when on (below) so the on voltage is determined only by the value of the current flowing and the MOSFET on-resistance:

 

SCRs are generally for DC

I'll quibble with that.
I believe most power SCRs are used in AC circuits (either inside a bridge or back-to-back in parallel).
They are rather difficult to use for DC switching since they can't be turned on expect by removing the current flow.
The small ones are used as logic latches in some circuit designs, however (one of my favorites is in a simple game show circuit where it latches for the first person that pushes his button).

 

i have to say this is marvellous, i came here with vague ideas, and already i know enough to actually follow this conversation, and form my own opinions. am very much looking forward to working with these devices.