HOMONYM: One of two or more words having the same sound and often the same spelling but different meanings. Examples: quail (cower), and quail (bird) fair (appearance), fair (county fair), and fair (reasonable).

HOMOPHONE: One of two or more words pronounced the same but different in meaning, origin, and sometimes spelling. Examples: cite, sight, and site; sea and see; your and you're; bow and bough.

HOMOGRAPH: One of two or more words spelled alike but different in origin, meaning, and sometimes pronunciation. Examples: bow of a ship, a bow and arrow, and a bow (deference/manners).

HETERONYM: One of two or more words that are spelled the same but that differ in pronunciation and meaning. Examples: bass (voice) and bass (fish); polish (shine) and Polish (from Poland); tear (rip) and tear (from eye).

Some excellent sites:

English Homophones
Homographs
The Heteronym Homepage
Shades of Meaning
One-trick Words
Alan Cooper's Homonyms
Self-study Homonym Quizzes
Homonym Game
Professor Darling's Notorious Confusables

Confusing Word Pairs

accede: stick to agreement exceed: surpass	accept: receive except: exclude	adapt: adjust adept: proficient
adverse: opposed averse: not interested	affect: change, influence effect: (v) to bring about (n) result, impression	all right: all right alright: outdated usage
allude: refer to indirectly elude: avoid	altar: place of worship alter: to change	appraise: value apprise: inform, notify
capital:seat of government; money capitol: building where legislative body meets	complement:something that completes (noun); to complete (verb). compliment: a flattering remark (noun); to praise (verb)	continual: happens frequently in time: close succession continuous: uninterrupted
emigration: moving from a country immigration: moving to a country	eminent: outstanding, revered imminent: threatening to happen soon	everyday: The everyday (ordinary) routine of working every day: Call me every day (each day) this week.
farther: refers to actual distance further: refers to figurative distance	imply: speaker implies (suggests) infer: hearer infers (perceives)	floe: large sheet of floating ice flow: move, run freely, circulate
lay: to set down, to place or put an item down lie: to recline	pray: to ask for by prayer or supplication prey: animal hunted or caught for food; victim	principal: first in authority; main participant; amount of a debt less interest principle: basic truth or assumption
raise: to cause to lift or to lift something rise: to ascend, move upward, or get up	real: adjective (real sugar) really: adverb (really wishing for...)	set: to place something somewhere sit: to be in a position of rest or to be seated
stationary: not moving: fixed stationery: writing material: letterhead, envelopes, etc.	suit: a set of clothes; legal action suite: number of items making up a set, series, or sequence	waiver: the giving up of a claim waver: to hesitate; also tremble or quaver

Confusing Word Triplets