Pioneer species refer to any species that are the first to recolonize an area after the original ecosystem has been disrupted. These species are the first in the chain of ecological succession that will eventually lead to a new climax community.
As the base of a food chain is always founded on the primary producers, plants are usually the pioneer species, particularly those that can live in unusual and often barren soils, or can gain foothold on rock. They tend to be very hardy, spread quickly, like direct sunlight, and are often noxious plants when they appear in climax communities or farmer's fields. They often have deep roots and may be able to fix nitrogen.
Any natural or man-made disaster of sufficient strength can disrupt an ecosystem, but the most common areas for pioneer species to move into are those affected by wild fire. Other causes may include volcanic activity, flooding, overgrazing, deforestation, plowing, shoreline erosion, etc.
Once the soil has been built up a bit and larger plants start providing shade (or pushy grasses take over), the pioneer species will start to become more marginalized -- this is not always the case, however. You may recognize the names of many pioneer species: gorse, thistle, raspberry, blackberry, white birch, scrub pine, sea grasses, and heath, among many others. Of course, the specific species will depend on the environment under discussion, and aquatic environments have their own pioneer species. Insects, fungi, and other groups also have their roles to play in pioneering a new environment, but they are less likely to be considered important pioneer species in the ecosystem; their populations may be very strongly influenced by their ability to act as a pioneer species, however.