I previously provided a primer on multidistrict litigation (MDL). Basically, MDL is appropriate when there are common questions of fact in two or more cases in different federal district courts. MDL consolidates the pretrial proceedings and places them in one forum. When the pretrial proceedings are complete, the cases are sent back to the courts from which they came.
MDL is a strategic consideration. In the paradigmatic example, a group of plaintiffs is not certified for a class action, and a corporate defendant tries to use MDL. In such a case, MDL is beneficial for both the defendant and plaintiffs, because it lowers litigation costs, and promotes efficiency and consistency. If the defendant's strategy is to delay litigation, which is common but ethically improper, MDL may not in the defendant's best interest.