TTL and CMOS are terms for particular families of electronic logic circuits.
A TTL output will generate two voltage levels: about 0.2V (low) and about 3.5V (high). It can generate the low voltage while conducting several mA from a more positive source (say a resistor between the output and a +5V power supply).
A TTL input expects a voltage greater than 2.4V but less than 5.5V to be applied to it for a high input (anything higher will blow it up). For a low input it expects an applied voltage of between 0V and 0.8V with 1.6mA of current being drawn out of the input pin.
CMOS is generally compatible with TTL, albeit with slightly different voltage levels. That said, in recent years the "standard" 5V power supply system has been replaced with progressively lower and lower voltages, particularly in very high performance computer chips. These days it would therefore be wise to qualify any statement about CMOS compatibility unless it is stated as TTL/CMOS (TTL is always a 5V system).