What is this weird Yahoo Instant Messenger (YIM) message?

You're talking about something similar to what I get sporadically from the Yahoo IM network. Something like this:

(I'm running the great multi-IM Adium program rather than the stock Yahoo app, which is why our windows look different. The message itself, however, looks the same unfortunately)

What you're seeing is something called spim, short for "spam via IM", and it's junk. This particular sequence is part of a larger spim message sent to me that's intended to use noise characters to prevent spim filters from recognizing that there's a URL buried in the message and to prevent it from seeing [misspelled] words like "nuddy pic" (which you can see buried in the message if you look closely. Just imagine that all the <#xxx> sequences are removed).

I won't decipher the URL here because it's not one I want to visit or promote and I'm sure you don't either, but this is just a daft attempt at sending an IM message that would make you visit their porn site. And I'm sure it's a failure.

Generally, I strongly recommend that you never approve a contact from someone you don't recognize in the IM world, just as you wouldn't share your house key with someone you weren't already quite familiar with in the real world. If you do connect with someone, you can also delete their contact using the real Yahoo IM application so that they can't bother you again.

As with email and the proliferation of spam and its adverse consequences, spim is an unfortunate fact of life in the IM world, but if you are diligent about only approving people you know, you should be pretty well insulated from its negative effects in your IM interaction.