Microscope slides can be classified into different categories. I want to give you a short overview here.
Permanent microscope slides
Permanent slides can be stored for a long time, provided that they were properly made. The specimens have to be properly conserved and cut into thin sections before they are mounted on the slide. Most permanent slides use a mounting medium which turns solid. These are the most stable. Properly made permanent slides can remain useful for a century. As a matter of fact, microscope slides from the 19th century are collector’s items.
It is also possible to make permanent slides with a liquid mounting medium. In this case the specimen is suspended in a small amount of liquid mounting medium and the cover glass is fixed to the slide with nail polish. Permanent slides using a liquid mounting medium are difficult to make and do have to be stored horizontally. I have also hear them to be referred to as semi-permanent slides.
Temporary microscope slides
Temporary microscope slides are made for the purpose of short time observations. After the observation session, they are discarded. In most cases the slides use a liquid mounting medium, such as water. Wet mounts can be temporary slides.
Wet mounted microscope slides
Wet mounts use a liquid mounting medium, such as water, immersion oil or glycerine. The different liquids have a different refractive index and are therefore suitable for different types of specimens. In most cases, wet mounts are temporary slides. It is possible, however, to make slides using a liquid mounting medium permanent, by holding the cover glass in place with glue or nail polish. The choice of the liquid mounting medium depends on the specimen. Water is most common medium and compatible with most biological specimens (after all, living things are made mostly of water). Glycerine has preserving characteristics and oil is used either for completely dry specimens (insect wings etc.) or hydrophobic specimens.
Dry mounted microscope slides
Dry mounted microscope slides do not use a mounting medium at all. The cover glass is placed directly on the dry specimen, which is surrounded by air. These can be useful, if there is a chemical incompatibility between the specimen and the mounting medium. Pollen and spores can be observed using a dry mount. Dry mounts produce a lower image quality, however. This is due to the strong difference in refractive index between the specimen and the surrounding air.
Heat fixed slides
No cover glass is used for slides. A suspension with bacteria is placed on a microscope slide and the water can evaporate at room temperature. The slide is then pulled through a gas flame to heat it up. Heat fixing makes the cells stick to the slide. Immersion oil is placed directly on the slide and the 100x oil immersion objective is used to observe the bacteria.