The difference between force and mass might best be found by looking at what each one is and then comparing them. Mass is something that any object or "thing" has. It speaks to how much "stuff" is in an object. All matter has mass and occupies a volume of space, and this is a definition of matter. Mass in a gravitational field expresses itself as weight, and an object with weight is just an object with mass that is being acted on by another object with mass. We have weight because we have mass and are being acted on by the mass of the earth. Note that anything with mass has gravity associated with it, and gravity is a force. Force is something that will cause a mass to accelerate. It is something that does work in that light. Certainly if we push on the side of building, it takes energy to do that, but nothing moves, and it could be argued that we haven't done any work. Force causes mass to accelerate in proportion to the amount of force applied and the mass being displaced. Mass speaks to what an object is made up of, and force is something that causes a mass to move. Links are provided to the Wikipedia posts on these two topics. Surf on over and read just the first paragraph of each post to get a handle on what's what.