Here is the idea behind bookmarking.
You have a website or blog post.
You go to one of these sites or include a button on your site.
And enter in your website or blog post URL.
Now the bookmarking site has a link to your site.
That is a backlink which is a good thing.
The more backlinks your site has, the higher your page rank "may" be on Google.
Some bookmark sites have a higher page rank and that is good for your page rank.
Some of these sites may be down or gone forever. I will add to it from some of my other bookmark sites I have here and there.
Almost all of these sites require registration and login to bookmark a site.
Create a login and keep it.
The fast way to submit to many social bookmarking sites is to use software or a website service to do the work for you.
The best are
BookMarking Demon - the most powerful and submits to 100+ social bookmark sites
Onlywire - easy
to use but only submits to about 20 social bookmark sites (and has been banned at some).
Social Marker - web interface, time consuming, but does build good backlinks and traffic to about 50 social bookmark sites.
Check out the list below. Some you have heard of others may be new.