Typically, paper/cardboard targets are either white or brown. Brown are targets - white are no-shoot. Black on targets indicates hard cover (i.e., hits in black are misses). Most practical shooting sports dictate that paper/cardboard targets are to be engaged with at least two rounds, unless otherwise specified by the CoF.
Falling steel has to fall to count. (This is why any stage involving steel in USPSA *must* be scored Comstock, so as not to force someone who has a hit-but-no-fall to take a miss.)
There are usually two major divisions of scoring rules in most disciplines - one that allows for make-up shots and one that mandates a certain round count exactly. In USPSA, the two are Comstock and Virginia. In IDPA, they are Vickers and Limited Vickers (sometimes people will say "Unlimited Vickers" to further enforce the distinction.)
Scoring schemes themselves also differ.
In USPSA, you are scored based on "hit factor" - points per second. Scoring zones are A, B, C, and D. Each has a point value assigned (B and C are the same) - the value depends on whether your gun + ammo combination makes Major or Minor power factor. Take the total score from the targets minus any penalties and then divide that by your time to get HF. You are then scored relative to others in your division for the stage to determine the amount of match points you will get from that course. Highest HF wins.
In IDPA, everything is turned into time-equivalent. IDPA targets have scoring regions of 0, 1, and 3. "Points" in IDPA are bad; each point adds one half-second to your overall time (i.e., overall score). Penalties may be added as points or directly as time, depending on the type (e.g., misses are points while procedurals are time). Lowest score wins.
Understanding how scoring works in the particular shooting discipline you are in is very important to being competitive. It allows you to decide how to better run courses. For example, if you are shooting an IDPA CoF and are reasonably certain that it would take you longer than one second to reliably place two shots in the 0-zone, it would be more beneficial (from a competitive aspect) to not take the extra time, since getting two 1's would be better for your overall score. USPSA scoring requires much more thought, which is also what makes it challenging to be competitive right off the bat - you have to learn both your abilities and how to best apply them in all kinds of situations.