Discussion on network abuse can be found here . MyCERT divides email abuse into several categories. The catagories are spamming, mailbombing, vulgar language, harrassment and chain letter. The explanation can be found here . The worse case would be when the email is sent to mailing lists, in which the email is forwarded to hundreds other users.
Most spam is for commercial advertising.
The recipients neither request nor expect the email. What makes people annoyed of spamming is because most of the costs are paid for by the recipients. Further reading on spam and unsolicited commercial email can be found here . There, you can set what kind of mail you wish to avoid. Most email softwares have this feature as well, try search in the help file.