
Here's something I posted about this a few years ago.  It's not fully
up to date with all the new variations (some of which are just different
manufacturer's synonyms):

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In practical terms, ignoring the technological details, this is my view
of the families (NB I am not a giant corporation, which influences my
views on things like availability and backward compatibility):

74	The original.  Speed good, power consumption fair.  Effectively
	obsolete now; use 74LS or later, except for a *very* few oddball
	functions like 7407 which are hard to find in newer families.

74H	Modification of 74 for higher speed, at the cost of higher
	power consumption.  Very obsolete; use 74F.

74L	Modification of 74 for lower power, at the cost of lower speed.
	Very obsolete; use CMOS.

74S	Later modification of 74 for even higher speed, at some cost in
	power consumption.  Effectively obsolete; use 74F.

74LS	Combination of 74L and 74S, for speed comparable to 74 with lower
	power consumption.  Best all-round TTL now, widest variety of
	devices.

74F	Fast as blazes, power not too bad.  The clear choice for high
	speed in TTL.  Availability and prices generally good.

74AS	Failed competitor to 74F, although a few 74AS parts do things
	that are hard to find in 74F and thus are still useful.

74ALS	Possible replacement for 74LS.  Generally souped up.  Still fairly
	new, availability and prices possibly a problem.

74C	Fairly old family, CMOS devices with TTL pinouts.  Competed with
	4000 series, not too successfully.  Obsolete; use 4000 or newer
	CMOS 74 families.

4000	(Thrown in as the major non-74 non-ECL logic family.)  The old CMOS
	family, still viable because of *very* wide range of devices, low
	power consumption, and wide range of supply voltages.  Not fast.
	Very forgiving and easy to work with (beware static electricity,
	but that comment applies to many other modern logic families too).
	There are neat devices in this family that exist in no other.  The
	clear choice when speed is not important.

74HC	A new attempt at 74-pinout CMOS.  Fast compared to old CMOS, power
	consumption often lower than TTL.  Possibly a good choice for
	general-purpose logic, assuming availability and affordability.
	CMOS logic levels, *not* TTL ones.  Beware very limited range of
	supply voltages compared to older CMOS, also major rise of power
	consumption at faster speeds.

74HCT	74HC with TTL logic levels.  Much the same comments as 74HC.  Read
	the fine print on things like power consumption -- TTL compatibility
	in CMOS involves some compromises.

10K	(Thrown in for speed freaks.)  The low end of ECL.  Various sources
	claim that it is *easier* to work with than super-fast TTL for
	serious high-speed work.  Less forgiving, though:  read and follow
	the rules or it won't work.  Availability to hobbyists limited,
	can be expensive.

100K	(For real speed freaks.)  Hot ECL.  Harder to handle than 10K, and
	inconvenient packages.  Much more useful datasheets, however.

As for compatibility between families:  the 74 families (except 74C and
74HC) are all more or less logic-level compatible, but how many 74X devices
you can drive from one 74Y output varies enormously with X and Y.  You just
have to read the specs and do the arithmetic.  74C and 74HC are compatible
with the others with a bit of hassle.  4000 compatibility can be a bit of
hassle or a lot of hassle depending on what supply voltage 4000 is using.
10K or 100K to anything else is considerable hassle.

Me?  I use 4000 and 74LS with a sprinkling of 74F.  74HC[T] and 10K are
interesting but I haven't used either significantly yet.