SEO or Search Engine Optimisation is a very complicated thing.
There are so many different aspects to SEO and it is vitally important for your online life.
For SEO there are some basic, major principles and then there are thousands of small details. The basics have been pretty much set in stone since the internet started. The thousands of small details have come into play since Google became the dominate search engine and started making all the rules.
You can have a look at what I mean by the thousands of small details by having a look through: