You may be wondering what css is. Well, it is cascading stylesheets. Still wanna know what it does? Well, it allows for more control over visual elements of your webpage than HTML itself does. What you can do is apply an innumerable number of visual elements to any tag on your web pages. You can change the font color, the font itself, the background color (and image), the element's padding, and even the location of the element. And that's still not saying everything that CSS can do.
It's a very simple thing to do, too. All you need to know is the word that performs the given change, and the values it will accept, and those same two things will work on just about every tag you apply them to. However, CSS isn't completely above the HTML that contains it. Major breaks can still interrupt a change that CSS made - if you start a new table row, any changes that aren't explicitly closed in the previous row probably won't make it to the new one. But it's a very small limitation that is easily avoided.
So how do you do it? It's simple. You can do it in multiple ways, such as these:
p {keyword:value; keyword:value}
font,p,tt{keyword:value; keyword:value}
p.big{keyword:value}
.red{keyword:red}
OK, the first one is the simplest of them. You simply tell the browser to take whatever's in a p tag and do the things in the brackets to it. You can also do that to multiple tags as the same time, like the second example. The third example is a class. This tells the browser only to apply the changes made in the brackets to p tags with the class big. It doesn't have to be the word big, though. It can be anything - just change the word big into whatever you want. The next one is also a class. But it can be applied to any tag. To use a class, just put the modifier class="class_name" into the appropriate tag.
Another thing to take note of is how the values are defined within the brackets. The keyword is the type of change being made, and the value is how the keyword will change the tag that it is applied to. The two are separated by a colon(:), and separate keyword:value pairs are separated by a semicolon(;).
And that's about it for the syntax of this tool. Now all you need to learn how to do is apply them to your web pages and learn the keywords themselves.