When you're starting to play around with CSS at the very beginning, like any other language, you have to get used to the syntax. Like any syntax, there are a bunch of little things you need to know. Some characters and the placement of them is very important and required for the CSS to work correctly. And some characters are more about clean looking code and generally followed standards but don't matter for the CSS to work.
First, so we have the terminology down:
A CSS ruleset consists of a selector and delcaration(s) wrapped in curly braces.
All CSS rulesets must have opening and closing curly braces:
... read the whole story at css-tricks.com.