How it is today:
There is the most matjure, biggeste and most expensive grid: Second Life.
But there are other grids - commercial and community driven.
The "old" storry:
Shoortly after you start discovering SL, you notice that many things are arround prims and place. There is the point where you want to get your own little place.
Please be aware of different prim/sqm rations and, that many places are just rental - so you donīt get back anything from your initial invest. But rental can be a very clever plan, if you have to play with the needed size , start a business or like that - prefered renting houses or condos is easy.
In the past, there was only "mainland". Mainland is owned by LL (linden labs). Today, there is mainland, and there are private SIMs.
These private SIMs usualy donīt need a paid type of SL account, just a free membership. All the financial stuff is done directly with them.
It is quite usual to pay via paypal.com , but a few landlords start now accepting L$ in world although.
Allways be sure to claim your price reduced "first land", get ~80% off on that place. if you ever want to resell it, we are interested, too.
An Note about the prices: be aware that land is influenced by similar factors like in real life. this can be infastructure arround, the view, the size and landscape and so on. At the moment the highest influences are that Lindens canīt put new land in as fast as user asking for it (itīs getting "rare", somehow) and people trying to trick other with fast money strategies. If you look carefull, a sqm of new land starts at ~4,9L$ - but others ask like 9L$ per sqm - and often those have a worse landscape and so on.
Although have a close (!) look at the covenant. There are zoning rules, building rules and much more.
A few links:
the biggest private land company, based on parcel sold: http://www.anshechung.com/
the biggest one based on sqm estates: http://www.azureislands.com
good prices, nice service: http://www.eworldmauritius.com/fantasyland/
black hat seems to keep an eye on building and generell look of the landscape: http://www.blackhatsims.com/
a few other big private SIM operators are: http://www.otherland-group.com/ - http://www.beachland-sl.com/
Did you ever wanted to get a overview about land and who owns it ?
This database is not 100% coverage, but growing and getting more actual on a day by day basis: http://www.dragonkeen.com/landmap/
A nice "how to" avoid junk on your lot: http://www.electricvenus.com/gaming/second-life/avoid-junk-on-your-land/
Did think about music on your land ? pick one from http://www.shoutcast.com/ , open the file with the editor and pick the url...