Spam is basically junk email usually in the form of advertisements or promotional material.  While advertising is a good idea and most people like to hear a bit of what is happening in the world spam is usually unwlecome.
Most people who have an email service use it for a reason such as business or private communication regularly.  A business doesn't have time to read junk mail and waste server time downloading the messages.  Both private and business email services are paid for by the user meaning the cost of spamming comes out of the users pocket.
Spam will fill your inbox very quickly and flood your mail server meaning longer download times and annoying messages when all you want is the important mail.  Your internet server is probably flooded with alot of spam (thousands of emails every day) that increases traffic and ultimately increased cost and annoying slow connections to the end user.
Advertising is ok in the real world because the target audience can usually ignore it and get by without to much trouble and the advertiser pays for the advertisement.  A little bit of spam would be acceptable but frankly there are way to many people doing it now.
Spammers are typically out for the money and don't care of the cost and annoyance to the recipient.  Even if you manage to filter out spam it still cost you money through download times and connection services.  Spamming is simply a cost you can't avoid.
There is hope though new laws are in place that make spamming illegal and several spammers have been taken through the courts and made too pay huge fines.  So if someone spams you let your service provider know and they may be able to help track down who it was and stop them doing it again.
Anyway the links on this site should help you find more information.

Well the answer is not much.  However doing what you can do will dramatically reduce the amount of junk you have to read.
Most email programs and service providers have filtering facilities meaning you can ask or set that mail from a certian person not be passed on to you or redirect it to your trash can or recycle bin (where most spam belongs anyway).
Simply filtering out things like XXX, XX or $$$ will reduce the amount of spam that gets through.   Then you can redirect it to another folder and or your trash can for checking later just in case an important message got caught up in the filtering.
Otherwise simple rules like never, ever respond to spam in any way inlcuding asking to be removed from a mailing list.  This will as I mentioned earlier confirm your addy is active meaning you will be spammed to death real soon.
Also don't leave your addy around anywaywhere especially newsgroups Spammers use special programs that scan for email addy's automatically.  If you do have to leave your addy somewhere include something funny in it like a stray symbol and an explaination of how to correct it for correct use e.g.
John@addy.com is the normal way of writing a typical addy but if you write it as @John@addy.com or something similar explaining to remove the first @ sign then spammers will get your addy as unreadable or not valid.  An easy way to fool their so called systems.

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