Roughly, muktze things are those that one can't or won't use on Shabas. There are several broad categories of this: if a particular object doesn't fall into one of these, it's not muktze.
- Valuable or fragile. If something is particularly valuable, or particularly fragile, so that handling it will ruin it, and it's not going to be used on Shabas, then it's muktze. An example sometimes given is a special knife used for slaughtering animals: it can easily be used for permitted uses on Shabas (contrast the "useful, but not for permitted activities" category below), but it won't be.
- Useless. If something is useless, it's muktze. An example sometimes given is raw meat, or a rock.
- Useful only in violation. If something can't be used without violating a halacha, it's muktze. An example sometimes given is chametz on Pesach.
- Useful, but not for permitted activities. If the only usual use for something is a forbidden activity, the object is muktze. An example sometimes given is a hammer. However, this category of muktze sometimes has an exemption if you have a valid use for the object for permitted activity.
- Set aside for its mitzva. If something is set aside for use for a mitzva, you can't use it for anything else: and if the mitzva is done, and it's Shabas, the object may be muktze also so that it can't be moved.
- Base. If something is a surface on which a muktze object is resting, the base may be muktze to the same degree as the upper object. An example sometimes given is a candlestick with a flame atop it.
- Muktze at the start of Shabas. If something was muktze at the start of Shabas it's generally muktze all Shabas long.
- Born. Something born or created on Shabas can't be moved until after Shabas.
- Deliberately set aside. Something set aside for non-use on Shabas can't be moved on Shabas. An example sometimes given is something put out, right before Shabas, to dry for a few days.
Of course, all of these have specific rules: this is just a general idea.
