Should I use a different address for every transaction? If yes, why? For safety? What happens I use the same address to get bitcoins from 2 different people, what happens?
The only reason why it is recommended to use a different address for every transaction, is for increased anonymity - it makes it more difficult to trace, and next to impossible to prove, that the recipient of transaction A is the same person as the recipient of transaction B.
If you publish an address to receive payments online, and keep it static, everyone can go to http://blockexplorer.com/ and see all the money you have received, and calculate how much money you got. If you change the address after every transaction, it makes it much more difficult - someone who have to constantly monitor your web page with the published address to know all your addresses you received money on. And if you keep the page with the published address non-public, such as that your customer has to log-in to his account on your system to see the address, that makes it practically impossible for third parties to monitor your income, find out how much money you have and what you spend it on.