To prevent excessive turnover in the minor league levels, each draftee costs $50,000. If the draftee does not stay on the selecting team's 25-man (major league) roster all season, the player must be offered back to his original team at half-price. Organizations may also draft players from AA or lower to play for their AAA affiliates (for $12,000) and may draft players from A teams or lower to play for their AA affiliates (for $4,000).
The Rule 5 draft has opened opportunities for teams to take other team's top prospects who may not be ready for the major leagues. A prominent recent example is Johan Santana, who was chosen in the 1999 Rule 5 draft by the Florida Marlins when the Houston Astrosdeclined to put him on their 40-man roster. After the Marlins traded Santana to the Minnesota Twins for minor leaguer Jared Camp, the Twins kept him on their roster for the 2000 season, despite the pitcher's subpar performance that season (6.49 earned run average) which was unsurprising given his youth and inexperience.
Having kept Santana in the major leagues for all of the 2000 season, the Twins subsequently had the right to option him to their minor league system. By 2002, the more fully developed Santana had returned to the major leagues and established himself as an above-average pitcher. From 2004 to the present, Santana has been considered by many to be the best pitcher in baseball, winning two Cy Young awards. Had he not been chosen in the Rule 5 draft, Santana likely would not have made his major league debut until the 2001 or the 2002 season with the Astros.
So that is a little bit of info on this draft and what it is for, there are exclusions to the draft as well. Read all about it at wikipedia here.