What are the script tags I HAVE TO insert into the `<head>` of my HTML???

Here is what I have:

```
<head>
<title>My Web Site</title>
<script type='text/javascript' src='http://code.jquery.com/jquery-1.9.1.min.js'></script>
<script src="http://ajax.googleapis.com/ajax/libs/jqueryui/1.9.1/jquery-ui.min.js"></script>
<link rel='stylesheet' type='text/css' href='http://code.jquery.com/ui/1.9.1/themes/base/jquery-ui.css'/>
<script type='text/javascript' src='scrollup.js'></script>
<script type='text/javascript' src='verticalsmoothscrolling.js'></script>
<link type="text/css" rel="stylesheet" href="stylesheet.css"/>
<link rel="shortcut icon" type="image/x-icon" href="favicon.ico" />
<link href='http://fonts.googleapis.com/css?family=Roboto+Slab:400,700' rel='stylesheet' type='text/css'>
<link href='http://fonts.googleapis.com/css?family=Fauna+One' rel='stylesheet' type='text/css'>
</head>

```

But if I delete first three script tags above and add this one instead: 

```
<script src="http://ajax.googleapis.com/ajax/libs/jquery/2.0.0/jquery.min.js"></script>

```

...everything works the same way (plus it looks clean and neat).

Can anyone explain me in simple words what each of the first three script tags does, and how do they differ from the one I added instead of those three??? Which one would you use?

I prefer the bottom solution but I am not sure if that's correct/best option???