There are three main types of lines on a tennis court:
The "baseline" serves as the outermost boundary on the tennis court. Any ball that lands behind the baseline is considered out-of-bounds. However, a ball may land on the baseline and still be fair game. The baseline also plays an important part during serves; the server must serve from behind the baseline, and their feet cannot pass in front of the baseline at any point prior to or during their serve. From baseline to baseline, a tennis court is 78 feet long.
On the center of the baseline, extending into the court, is the center mark. The center mark denotes the center of the court, and helps a player know the side on which they are playing, especially when they are returning a serve.
The two service lines (and the singles sidelines) form the boundaries of what are known as the "service boxes." There are four of them on the court. The service boxes are important because the server must bounce their ball inside the service box diagonally across from the point at which they serve. If they serve on the left side of the baseline, they must land their serve in the right service box. Players serve on alternating sides of the centermark, switching after each point.
The center service line is 42 feet long (21 feet on either side of the net), and the other service lines are 27 feet wide.
The singles and doubles sidelines form the side boundaries of the tennis court. In singles games, the sideline lies 13.5 feet away from the center of the court, and extends from baseline to baseline. In a singles game, if a ball lands outside of the singles sideline, it is out-of-bounds. Like the baseline, if a ball lands on the line (but not outside of it), it is still an in-play ball.
For doubles games, the doubles sidelines are used. These sidelines rest 18 feet from either side of the centermark, making the doubles court a total of nine feet wider than the singles court.
Even though there are different sidelines for singles and doubles play, the service box always uses the singles sideline as a defining boundary, never the doubles sideline.