Phonology

Phonology is a system of contrastive relationships among the speech sounds that constitute the basic components of a language. Phonology includes an assortment of sounds and their features, and rules which specify how sounds interact with each other. Analyses of the sound patterns of a certain language by figuring out which phonetic sounds are important and explaining how these sounds are actually heard by native speakers. There are two major branches of phonology, phonemics and phonetics. Phonemics is the narrow study of sounds for example the 'P' in 'speak' where as phonetics is the broader study of sounds for example the ‘P’ in ‘speak’ and ‘peak’.

Some of the essential terms in Phonology are Phonemes, allophones, minimal pairs, distinctive features, syllable structure, phonotactics, morphophonemics. These terms describe distinctive or contrastive sounds, difference between pairs of words and their phonetic properties and meanings.

Loudness
volume of speech can determine emotions.
a) GO AWAY!
b) I think, we should leave them alone.
tone is indirectly affected. 

Stress

In linguistics, stress is the relative emphasis that may be given to certain syllables in a word, or to certain words in a phrase or sentence.

1. The hot dogs run to the lake. (adjective - stress is on the  second syllable)
2. Dogs will run when frightened. (verb - stress is on the first syllable) 

 Phonology allows us to make up new words that "sound right" and also allows us to add sounds to make plurals, paste tense, etc. Additionally allows us to know what is and what is not a sounds in our own language.

Further information-http://www.youtube.com/watch?v=_69ATDAomLc




sources:
http://www-01.sil.org/linguistics/GlossaryOfLinguisticTerms/WhatIsPhonology.htm
http://www.teachit.co.uk/armoore/lang/phonology.htm
http://www.dmoz.org/Science/Social_Sciences/Linguistics/Phonetics_and_Phonology/
http://ocw.mit.edu/courses/linguistics-and-philosophy/24-910-topics-in-linguistic-theory-laboratory-phonology-spring-2007/24-910s07.jpg