Tabs hold information.
The active tab is the tab that is currently selected. Only one tab may be active at a time. Many preferences, especially general preferences specify functionality based on which tab is active.
Tab List and Tab Bar
By specifying the preference General -> Tabs of left on or off, you can choose whether to have a tab list or a tab bar. If you are in many channels, a tab list is generally better than a tab bar. The Tab bar is located right above the chat pane. The tab list is located to the left of the chat pane.
Alt+UpArrow toggles between "Tabs on left" and on top (see also "Left tabs width" etc in Prefs/General).
Note: Alt+Arrows doesn't work in Opera or Safari(?). In Opera you can use AltGr (right of the space bar) instead - at least using a German keyboard.