Later you may want to learn JavaScript. You should. For now you only need to know what it is.
A long time ago in Web terms Netscape made it possible to trigger automated things to happen on Web pages. Originally, it was called Mocha, LiveWire and LiveScript but was renamed to make it sound like it had something to do with Java even though it hasn't.
Originally JavaScript was quite limited. It went through a complicated history of conflicting development but is now starting to be fairly standard across all browsers. It allows you to change Web pages even after they have loaded (DHTML), make checks on data entered into forms on Web pages and create many additional Web page features such as clocks, calendars and silly tricks with mouse cursors!
If you see parts of a page which change it is probably JavaScript (or animation).
