These terms come up in species descriptions.  A plant's growth form (annual, biennial, perennial) also impacts the best method of control for that species.  

Annual: A plant which completes its life cycle in one season, spring through fall.

Biennial: Plants that require 2 seasons to complete life cycle. They typically germinate in late summer of year one, then over-winter, flower, and set seed by mid-summer of the following year.

Perennial: Plants that live 3 or more years.

Bract: A small structure below the flower; can be leaf-like or spiny

Pappus: The “parachute” of a seed.

Rhizome: An underground stem, usually lateral, sending out shoots above ground and roots below ground.  The photo on page  12 of Canada thistle roots depicts rhizomes.

Taproot: One large, central root from which other roots sprout laterally.  A carrot is an example of a taproot

Fibrous Roots:  Many small, thin roots of similar size.  Most grass roots are fibrous roots.