a GEISE is basically a 'magical', or in this case ethereal, prohibition or law. A good way to think of a Geise is the cost of the ethereal abilities. It is generally related in some way to the type of ethereal creature or magical ability. Geise generally come in four forms: Contracts, Limits, Constraints, and Provisos.
The first type, called a PROVISO, stipulates that a certain action or type of action cannot be taken. This includes provisions to never eat the flesh of an animal that walked on four legs, a prohibition against wearing a certain color or saying a certain word, etc. Generally it is a small, simple thing, easy to avoid, and easy to forget about. It can also include something that the person loves or feels a constant need to do. However, directly after breaking the Proviso, the person experiences great pain, extreme bad luck (including loss of loved ones, memory, health, etc.), or even, gruesome death.
The CONSTRAINT actually eliminates a certain behavior, so that the bearer of the Geise literally doesn't have the ability to do that anymore. This can include a vampire's inability to enter a house without verbal permission, an inability to say anything that is not true, an inability to kill, etc. They can also be silly, including the inability to touch fingernails, but in general tend to be a constant annoyance to the person who bears it.
Where a constraint limits taking certain actions, a LIMIT eliminates abilities. For example, a Constraint might eliminate a person from eating red fruit, but a Limit would eliminate the ability to even see the fruit. Limits include not being able to see certain colors (so that anything that color is invisible), or a loss of intelligence and emotional control during transformation (like a werewolf). The X-men Rogue's (r) inability to touch someone without almost killing them, or Cyclopes (r) inability to see someone without protective lenses, could be considered Limits.
Similar to the other three, a CONTRACT is an action that must be taken. There are Proviso Contracts, where a certain action needs to be taken to avoid pain; Constraint Contracts, where the person performs the Geise whether they want to or not; and Limit Contracts, where abilities have limits (such as being able to change into any type of mammal, but always being green in color.