This article is somewhat more technical than usual but it addresses what is probably the most common problem that people experience with Joomla. Joomla can be successfully used with a number of hosts all of whom have different setups. But once you start installing extensions, you may encounter difficulties with file permissions. If you're not technically minded, you don't know where to begin to solve these problems and spend hours tearing your hair out trying things that don't work or even wrecking the rest of your site.
If you encounter this, ask your host if your server is using suPHP. I won't go into details except to say that this approach will not only make your Joomla life much easier, it will make your site more secure too. If they can't help you, it might be time to look elsewhere. If you are technically minded, you may wish to read this post for more details.