From the SMTnet Library Terms & Definitions [er whatever they call it] �
Emulsion. A material that suppliers build-up on a printing screen to block portions of the screen. The un-blocked (open) portion of the screen define the pattern for depositing solder paste on the printed circuit board.
Just to expand � We use stencils and screens to apply liquids [glue, solder paste, solder mask] to our boards.
* Stencil. A sheet of material with openings corresponding to the SMD pad pattern on a printed circuit board. The most common material is stainless steel. Typical thickness ranges from 0.005 to 0.008 inch. * Screen. A screen mesh covered by a emulsion. The emulsion has openings that match the land pattern of the printed circuit board. During printing adhesive or solder paste is forced through these openings onto the printed circuit board.