CrossPosting is the practice of asking the same question in more than one place at the same time. Such as asking in both the Database Q & A forum and the ASP Q & A forum.
It is evil because it wastes time and energy that could be used better for other things.
For example, on occasion, I have spent 15 to 30 minutes answering a question in one forum...only to change forums and find out the question was answered 10 minutes before I started!
When you crosspost, you waste the time of those who answer you and the time of all the other people who have to read your post in the wrong forum.
I, personally, allow everyone one or two crossposts, but if anybody does it continuously, I stop answering that person.
Crossposting has always (more than 10 years now!) been considered to be bad Netiquette. So learn the rules and be polite to others and don't waste our time...In other words, don't crosspost.
NOTE: If you don't get an answer in one forum within a reasonable time (defined as from two hours to eight hours, depending on how busy the forum is when you post), then it's probably okay to crosspost--or even re-post to the same forum--providing that you put a note in the second post that mentions the first one (both by forum name and approximate time of posting). That way, anybody who might want to answer you can first check to see if you've finally gotten an answer to the original posting.
DO practice good Netiquette. It makes the entire forum experience much more pleasant for everyone.