The basic difference (most of the time) between AFFECT and EFFECT is that AFFECT is a verb, and EFFECT is a noun. About 90 percent of the time, that definition will serve you well.  Here are some example sentences:

 My uncle's stroke affected his ability to speak clearly.

My uncle's stroke had an effect on his ability to speak clearly.

 

The slow economy is affecting the election campaigns.

One definite effect of the slow economy is cuts in local government.

 In a less well-known usage, EFFECT can be used as a verb that means "to bring about" something. I came across this usage recently in a fascinating article about the Great Sunflower Project launched by San Francisco State University associate biology professor Gretchen LeBuhn. LeBuhn has enlisted volunteers across the country to plant sunflowers and other bee-friendly flowers in their yards, then count the number of bees that visit during two 15-minute observation periods each month. She wants to find out why bee populations have plummeted in recent years and what can be done to increase those populations.

Here is the sentence:

I used to despair about our ability to affect change.

I do not know how Gretchen LeBuhn would have spelled this word if she had written her comment rather than spoken it, but the writer who quoted her in the Fall/Winter issue of SF State spelled it AFFECT, which would suggest the idea of "having an impact on" change. I think LeBuhn was probably referring to "bringing about" change, She went on to say in her interview that, "This (the Great Sunflower Project) restored hope in me that we as a society can do some things to really improve the world." I believe the sentence should have been written this way:

I used to despair about our ability to  ef fect change.

Here is another example of an error with the less common usage of AFFECT and EFFECT. It appeared in a comment on one of the LinkedIn forums I read:

I'd say that YES misspelling and bad grammar effect credibility of a company. 

What this writer meant to say was that misspelling and bad grammar have a bad EFFECT on the credibility of a company.  (I agree completely with what she meant to say.) However, as worded, it sounds as if misspelling and bad grammar "bring about" credibility, which is the exact opposite of what she meant.  It should be written one of these two ways:

I'd say that YES misspelling and bad grammar can affect the credibility of a company. 

I'd say that YES misspelling and bad grammar can have a negative effect on the credibility of a company. 

If you would like to know more about the Great Sunflower Project, you can visit www.greatsunflower.org.

While preparing a workshop on Usage Glitches, I visited Microsoft Office's "clip art on line" site this week.  I wanted to find a graphic of letters and envelopes and another one of an exercise bike.  My purpose was to illustrate the point that STATIONERY means one thing and STATIONARY another.

I was quite surprised to discover that, when I keyed in either word, graphics for both meanings came up.  Whoops! One of these words, STATIONERY, is a noun and refers to paper and envelopes still used by many of us for snail mail correspondence.  The other word, STATIONARY, is an adjective and refers to something that is not moving.  The two words are not interchangeable, as the Microsoft "clip art on line" site would suggest. However, most search engines allow for the fact that many people do not know the difference!

Here are some reminder examples of proper usage:

I prefer cream white STATIONERY with a gold or silver border.

Our new local STATIONERY store carries many brands, including Crane.

 

John rides a STATIONARY bicycle every morning before work.

The population of Sunnyville has remained STATIONARY for the past ten years.