Databases

As one cannot imagine a car without an engine so one cannot imagine modern small or huge web applications without databases.

An useful database will have to :
1- store every single piece of relevant data.
This means, not only product details or access details for customers but also a lot of user behaviour related details like at what time of day will the most users login, what will an user most likely buy, how did an anvertising campaign change the user behaviour and so on.
Tracking and using all this data can make a difference on a long run.

2- return results fast.
An useful database will have not only to hold a lot of data but also be designed so it returns any results FAST.

3- be scalable.
It is very important to design a database having in mind that your application may one day have such a success that you'll need many servers. Designing the database so it can be easily spread on many servers at the right time can make a very big difference.