Second life is free to everyone, you can gain Linden Dollars (L$) a in world currency buy adding a payment source like paypal, but many people who have been playing for years never pay for money.
The purpose of money is to buy/rent things as you do with real life currency. You can buy/rent land, which you have control of and use as a place as home, somewhere to do business or a public location. You can also buy lots of things, like clothing, skin, body shapes, cars, houses and much more, have a look at http://marketplace.secondlife.com/ for a good example of what you can get.
There are things you can get without using lindens, but they are limited, however as second life is mainly a social game there isn't any reason you have to them.
You can also 'earn' Lindens by people paying you, there are many SL jobs or you can sell your creations.
http://wiki.secondlife.com/wiki/Guide_to_Jobs_in_Second_Life
If you decide to buy lindens, the exchange rate is around L$260 per 1 USD, not including Fees, and you can also transfer Lindens back into USD, so it is possible to make real life Money from Second life.
There are two types of account, basic and premium.
The benefits of premium are:
* Ability to buy mainland
* 512msq of free land or a Linden Home
* 24/7 Technical support
* L$300 weekly to spend as you like
You can still buy a island or rent anywhere with a free account.
The inbuilt voice changer can be useful as it is easy to use, however, it is a lot of money and not very good.
There are some much better voice changing software on the internet, some free and some for purchase, that is more authentic sounding.
Second life is worth the time put into it, with or with out money people can have lots of fun, and even earn a little pocket money if your willing to put the time in. It is more a social universe with the ability to explore, shop, roleplay in 3D and be whoever you want and meet people from all around the world!