Speech Recognition Engines ("SRE"s) are made up of the following components:

• Language Model or Grammar - Language Models contain a very large list of words and their probability of occurrence in a given sequence.  They are used in dictation applications.  Grammars are a much smaller file containing sets of predefined combinations of words.  Grammars are used in IVR or desktop Command and Control applications.   Each word in a Language Model or Grammar has an associated list of phonemes (which correspond to the distinct sounds that make up a word).
• Acoustic Model - Contains a statistical representation of the distinct sounds that make up each word in the Language Model or Grammar.  Each distinct sound corresponds to a phoneme.
  
• Decoder - Software program that takes the sounds spoken by a user and searches the Acoustic Model for the equivalent sounds.  When a match is made, the Decoder determines the phoneme corresponding to the sound.  It keeps track of the matching phonemes until it reaches a pause in the users speech.  It then searches the Language Model or Grammar file for the equivalent series of phonemes.  If a match is made it returns the text of the corresponding word or phrase to the calling program. 

A Speech Recognition System ('SRS') on a desktop computer does what a typical user of speech recognition would expect it to do: you speak a command into your microphone and the computer does something, or you dictate something to the computer and it types it out the corresponding text on your screen. 

An SRS typically includes a Speech Recognition Engine and a Dialog Manager (and may or may not include a Text to Speech Engine).