Define symbolism as used in literature, and what are some universal examples of symbols most people understand?Also, why do writers and poets use...

A symbol, whether in a story or "real life," is when one thing stands for another. That's a pretty basic definition.  If I have a shoe on my mantle because it reminds me of my time as a long distance runner, well, the shoe has become a symbol of running for me.  People have lots of symbols without always realizing it.  Sometimes, after being out of town for a long time, you might see a particular billboard or restaurant that reminds you of home.  That billboard (or whatever) has become a symbol of your home town.

A universal symbol most people seem to understand is the flag. When someone sees the flag, they think of America (or whatever country they happen to be from.) The Statue of Liberty is another good example. It is a universal symbol of freedom.

Poets and writers use symbols in their writing, sometimes knowing they are doing it, and sometimes people only find them after the fact. I don't know too many fiction writers who sit and think about how to put symbols into their writing. Mostly it happens because someone reads the story, gets a "theme" out of it, and then finds objects or people in the story that represent that theme.

I think poets are more likely to put symbols into their writing because their type of writing is different. It deals more with emotion, imagery, and subtleties. Instead of talking about the death of a romance, for example, a poet could discuss how the petals fall off of a rose. The rose becomes a symbol for love. See how that works?

Poets use it so that they can discuss familiar topics, like love, from fresh angles.  I mean, if every poem about failed romance had to have people in it, well, they would get boring after about the millionth one.  Fiction writers, like I said, are largely unconscious about the symbols in their writing.  People mostly make the connections after the fact.  Of course, there are exceptions, and I am sure some writers make a point of putting certain objects in their stories to represent other things...I just don't think they set out to do it.  It more kind of just happens than it does in poetry.

At least, thems my opinions!