spamming is usually from bots made to post their link on as many forums as possible, but it can also be done by people.

generally things that admins see as spam is:

1. joining a forum just to post links advertising a website
2. posting loads of pointless low quality posts just to increase post count
3. using private messages to advertise your website to other members

there are many ways to prevent and protect your forum from spam:

on the registration page, there are things you can add on to prevent bots from registering, such as captcha and adding an extra profile field with a question in it, that is easy enough for a human to answer, but a bot wouldnt be able to. this will stop most spambots from registering.
for the spambots that get past the registration process, and human spammers, you can use the newly registered users group, setting it so the first post of every first user has to be approved by a staff member.

Well said by Banana. Because a human being can easily read a question rather than a spam bot either it is Captcha or any question.

yes, those 3 are all spammer's factor, its too much difficult to tackle themselves, in recent days when paid CAPTCHA solver comes, you know its really quick and easy to overcome by automation. But extra profile field and review the very first post is really helpful to avoid them. Thanks for sharing.