It's people's personal journals but there are also communities that you can join and post in.
For some of them (i.e. the mac community, which I presume you'd be interested in), posts are sometimes friends-only, which means you have to join LJ and then join the community you're interested in to see the friends only posts.
It's free unless you want to pay for more services, it's customizable, lots of good people and pretty much anything you might be interested in is represented there.
Go to
www.livejournal.com and somewhere on that front page, it will have a link to create a journal - it guides you through the process. But if you have any questions, you can either PM me here or email me (I'm jasper17 there too) and I'd be glad to help.
EDIT - sorry - missed the part where you said you'd already created a journal. You can search by interest to find communities and people you're interested in, join the communities, add people to your friends list and post in both places. Kind of like a forum but not.