The basic concept in ExpressionEngine is that of a template. Go to any ExpressionEngine-powered website and you will undoubtedly be looking at a template. Templates are what the outside world sees.
At its most basic, a template in ExpressionEngine is a HTML (or CSS or JavaScript) file. If we wanted to, we could use a template exactly like a HTML file, without any problems. We could create an entire website without ever using any other part of ExpressionEngine.
However, we can take templates a lot further than that. By using ExpressionEngine tags inside our templates, we can take advantage of all the features of ExpressionEngine and combine it with all the flexibility that HTML and CSS offers in terms of layout and design. We are not limited to pre-defined 'cookie-cutter' templates that have been carefully adapted to work with ExpressionEngine. This is why ExpressionEngine is very popular with website designers.
On the flip side, this is also why there is such a learning...