WHO MAKES THE RULES?
IFAB
The International Football Association Board was established in 1886 in part to standardize the rules in a "Laws of the Game" manual. FIFA joined IFAB in 1913. NWSL Competition Rules are posted prior to each season.
QUICK EXPLAINERS
Offside
A player is offside if they are closer to the goal than the second-to-last defender (the keeper being the last defender) when the ball is passed to an advancing player. These videos are great explainers.
Offside (More advanced)
Out of bounds
The field of play, described in Law 1, includes goal lines (think end lines) and touch lines (think sidelines). When the ball is wholly out of bounds either a throw in (touch lines), goal kick or corner kick (goal lines) is awarded. See Laws 16 Goal Kick or Law 17 Corner Kick.
Additional Time
Since international soccer features a running clock (time doesn't stop when a whistle blows), additional time gets added at the end of each half of play to account for things like substitutions, celebrations for goals, disciplinary action, injuries (players rolling around the ground), etc. This is also referred to as stoppage time.
Extra Time
(Think overtime.) Extra time is a method for trying to determine the outcome of a match. At the end of two 45-minute halves if the score it tied, two equal extra periods (15 minutes for pro or international play) are added. ​
Foul vs. Yellow Card vs. Red Card
Basically, severity of the foul determines color of the card. See Law 12, Fouls and Misconduct. A sending-off offense (red) includes these behaviors:
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denying the opposing team a goal or an obvious goal-scoring opportunity by a handball offense
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denying a goal or an obvious goal-scoring opportunity to an opponent whose overall movement is towards the offender’s goal in the penalty area by an offense punishable by a free kick (unless as outlined below).
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serious foul play
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biting or spitting at someone
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violent conduct
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using offensive, insulting or abusive language and/or action(s)
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receiving a second caution (2 yellow cards) in the same match
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entering the video operation room.
Penalty Kicks (PK)
A penalty kick is awarded if a player commits a direct free kick offense inside their penalty area or off the field as part of play as outlined in Laws 12 and 13.
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Generally only used during tournament play, a penalty kick (PK) system may be used if a match results in a tie and a winner needs to be determined.
Substitutions
Youth and college soccer allow for many more substitutions than do pro or international leagues. (It would be difficult to keep enough players on teams to scrimmage during practice, otherwise.)
The use of return substitutions is only permitted in youth or other grassroots football endeavors.
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The NWSL uses international rules and so there are a maximum of five substitutions across three separate occasions (aka windows) determined by each team's coach during normal regulation time, plus "concussion" substitutes, if required.
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If a team uses up either substitutions or the windows allotted during a match, a team may be forced to play "man down" for remaining time in any contest.