This set of posts comes from Pastor Mark Driscoll and his wife Grace. The sermon series The Peasant Princess brought up dozens of questions that they felt they should respond to. The result is this blog series: Christian Sex: Frank Answers to Honest Questions.
Manual stimulation is when a hand, finger, tongue, or other part of the body is used to pleasure one's spouse. Because this is done together for building oneness and pleasure, it is to be encouraged but not forced. Masturbation is doing the same kinds of things to oneself, which is acceptable if your spouse is present and consents. However, it is to be discouraged if your spouse is not present and/or does not consent because it does not build oneness.