There are really no disadvantages to using a database. I use a MySQL database and PHP to interpret the information. The advantages of a database are numerous.
1. You can keep a LOT of information strored for your website in a database and have it accessed as needed. For instance, someone can search your database for a certain item, name, etc.
2. You can store actual HTML pages in the database so instead of making 10 html pages, you create one PHP page and pull the data needed pertaining to the users criteria.
3. You can not only store information, but sort it as well. For instance, get information for a certain state or person only.
The best way to describe a database is its like a telephone book. The book is the database, the whitepages are a table and the listings are rows in the table. So, you would have a column for lastname, first name, address, city, state, zip, etc. Then each persons information is added to the table row. Then, if someone wants listings for Jones, the PHP pulls the last names that match Jones and all the information associated with it. Or just the letter L and so on.
The phone book is just an example and it can apply to just about anything. A clothing store, cd collection, etc.
In conclusion, its an awesome web tool for any site and its well worth looking into. I would suggest using PHP and MySQL because they're both free, fairly easy to learn and theres a LOT of support on the web for both.
It really depends on what you are trying to do, and versus what. MySQL versus Access? MySQL versus Oracle? MySQL versus DB2? MySQL versus a spreadsheet? MySQL is good for web apps, integrates nicely with PHP, and as its community server is free