Extracts of plant material contain more concentrated amounts of the aromatic, functional or active components of the original material.

There are various ways to extract; the method chosen depends on the material, what you are extracting and/or what form you want the final extract to take. 

Methods for extraction include Expression (or Cold-Pressing), Maceration (Chopping/grinding the original material), Steam Distillation, CO2 Distillation, Solvent Extraction.

Herbal and Spice Extracts are used to impart flavour, used in the production of flavours, or used as a functional food ingredients. There are also technical reasons for inclusion such as natural foaming agents, or bittering agents for example

There are several forms , these tend to be native based on the material being extracted:

Solid Extract (SE) - Highly viscose but is still of a pouring consistency.
Fluid Extract (FE) - This is a less viscous, more fluid form.
Powder Extract (PE) - Individual free-flowing granules to smaller particles.

Extracts are often stated with the percentage of active ingredient following the name e.g. 6%, 10%, to prove that it is a multiple of the amount in the original raw material. Alternatively a ratio may be present (presence of a ratio implies that the product is an extract) which shows the concentration of the product in comparison to the original material e.g. 4:1, 10:1.