David Landry wrote this in 2007. I have refreshed it.

There are 40 active players on each MLB team’s roster. Of those 40 players, only 25 players are on the active roster at the completion of spring training. The remaining 15 players are typically reassigned to a team’s minor league affiliates.

In September the active roster expands to 40 players. This provides top prospects with an opportunity to play at a major league level. Players who appear in one or two games in September and who never return are said to have had a “cup of coffee” in the majors. The classic example of this is Moonlight Graham from Field of Dreams. He appeared in one game for the NY Giants and had no at bats.

The Line-up
Prior to each game, the manager provides the umpire with a line-up card that names the players at every position and lays out the batting order. In the National League, there are nine players on the line-up – the pitcher, catcher, four infielders (first baseman, second baseman, third baseman, and shortstop), and three outfielders (left fielder, center fielder and right fielder). In the American League, the pitcher does not bat, so there is a tenth player, the designated hitter.

The remainder of the 25 player roster varies slightly by team, but typically consists of four more starting pitchers, five to seven relief pitchers, one or two back-up catchers, two back-up infielders and two back-up outfielders.

Substitution
If a player is replaced in the field or in their place in the batting order, they cannot return to the game. A batter brought in solely for the purpose of batting for another player is referred to as a pinch hitter. Players who substitute at more than one infield position are called utility infielders. Similarly, there are utility outfielders. Pitchers who come into the game after a starting pitcher are called relief pitchers. A closer is a relief pitcher that specializes in coming into games in the ninth inning when his team is leading. His job is to close down the other team.


The Disabled List
If a player on the 25 player roster is injured and officially placed on the disabled list, that player is unavailable to the team for a minimum of 15 days while recovering from that injury. There is a 15 day disabled list and a 60 day disabled list for more seriously injured players.

Injured players are sometimes listed as day-to-day. This happens when the team does not want to lose the player for 15 days. The start date can be made retroactive if the injury is determined to be longer term after a player has been day-to-day.

The Rule 5 Draft
Players who are not on a team's 40 player roster who have at least three years of service as of November 20th can be drafted by other teams for a fee of $50,000. This prevents teams from stockpiling prospects. If a player is drafted under Rule 5, they must appear on the drafting team’s 25 player roster for the next year. If that player is not kept on the 25 player roster, the original team can draft him back for $25,000. This draft has yielded excellent players in the past, including Hall of Famer Roberto Clemente and Cy Young Award winner Johan Santana.


Shoeless Joe
Since I mentioned Moonlight Graham above, I want to recommend the book “Shoeless Joe” by W.P. Kinsella. It served as the basis for Field of Dreams and is very well written. Any book with baseball and J.D. Salinger is worth reading.