Portable air conditioners are larger than window units and sit on the floor. Window air conditioners, on the other hand, are units that rest in a secured open window.
Portable air conditioners transport easily from one room to another while window units take more effort. Window units are quieter since they direct their noise outside. However, in the winter, you must bring the unit in from the window.
The best air conditioner for you depends on your situation. Portable units offer more versatility but they also take up a good deal of space. Window units cost less but provide less mobility.