Species (organism level) - An organism which has certain characteristics of a given population and is potentially capable of breeding with the same population defines a member of a species. This definition does not apply to asexually reproducing forms of life such as Monera, Protista, etc. Species can be considered the lowest (most specific) area of biological classification, but lower groupings are sometimes employed (e.g., subspecies, variety, race).

Population - This term applies to organisms of the same species which inhabit a specific area.

Community - A community is an aggregate of populations of different plant and animal species occurring within a given area.

Habitat - A habitat is an area where a specific animal or plant is capable of living and growing; usually characterized by physical features, or the presence of certain animals or plants.

Niche - This term applies to an organism's physical location and, most importantly, functional role (much like an occupation; what the organism specifically does) within an ecosystem.

Ecosystem - a functioning natural unit with interacting biotic and abiotic components in a system whose boundaries are determined by the cycles and flux of energy, materials and organisms.

Ecotone - An ecotone is a boundary ecosystem, specifically the ecosystem which forms as a transition between two adjacent systems. It may possess characteristics of both bordering ecosystems, while developing a suite of its own characteristics. Examples: Riparian zones, coastal forests.

Biosphere - This is the surface zone of the planet earth, extending from within the earth's crust up into the atmosphere, within which all known life forms exist.

Example: Ecological Levels
Species: Cervus elaphus (Roosevelt elk)
Population: The sum total of all the elk in a given herd (e.g., migrating through the Greater Yellowstone Ecosystem)
Community: The elk, and other populations associated with them (e.g., wolf, grizzly, bison)
Habitat: The place where these elk live -- the open timberlands of Yellowstone NP
Niche: Primarily browses on shrubs, broadleafs, and new growth of conifers
Ecosystem: Yellowstone NP and surrounding forest, shrubland and grassland, extending to the limit of the elk range (if we define it from the elk's perspective)

From EPA Watershed Academy - Ecology - The Biological Setting - Table of definitions