Lacrosse can get pretty complicated, but this simple information can help anyone to understand this sport that's gaining traction in the U.S.
First let's look at the layout of the field. Each team has 10 players on the field at any time (unless there has been a penalty). There always needs to be three players on the defensive side, excluding the goalie, and three players on the offensive side of the field. The three midfield players, or middies, rotate between playing offense and defense.
Example of a lacrosse field (Courtesy of http://ijournal.us/articles/issue-31/ijn112/case-club-lacrosse-ca-community-colleges.html)
Players use sticks with a 'head' on top that have nets to catch and throw the ball. There are two types of sticks: long poles, which are used mainly by defensive players, and short poles, which are used mostly by middies and attack players. Players wear padding to keep their arms, shoulders, chests and heads safe, since hitting with these sticks is allowed to a certain degree.
This is a basic layout of the lacrosse equipment needed. (Courtesy of https://genoalacrosse.wordpress.com/lax-101/)
Each game starts off with what are called 'face-offs.' One player from each team lines up in the center of the field, where a ball is waiting on the ground. When the referee blows his whistle each team fights to gain possession of the ball. Two players from each team are able to come in to assist their teammate in the face-off, but they must start at the 'wing area' (see map above). Face-offs also occur after either team scores a goal.
Players are allowed to substitute throughout the game in a way similar to a hockey game. The player entering the game must wait for the current player to exit the field before making the substitution.
There are a few other reasons why a player would get called for a penalty, such as a hit to the helmet with a stick, cross-checking a player with a stick (this is where one player hits another player with the stick and their hands are not touching) or being off-sides. Game officials throw a yellow flag whenever an infraction is committed, but the play is allowed to continue until possession has changed to enforce the penalty.
The above play was a penalty, because the hit was high on the chest of the other player. When a player gets called for a penalty, the other team will have a 'man-up' situation, which gives it an advantage. The times for penalties ranges between 30 seconds and three minutes, depending on the nature of the penalty.
The main objective is to score the ball in the goal on either side of the field. There is some space behind the goal to allow players to run in front or behind the goal while moving the ball around. When a shot misses the net and travels out of bounds, the team that has a player closest to the ball when it exits the field is given possession.
There is much more to know about lacrosse, but with this information one will be able to go to a game and understand what is happening.