On the top left corner of the map, you have the zoom in/zoom out buttons. You can also just use the scroll in/out on your mouse.
Aircraft ICONs
The aircraft icons are displayed all over the map, depending on its latest received position. Each aircraft is displayed in one of 3 colours that describes it's situation:
aircraft information
While moving over the aircraft icon, the tooltip will show up to give you more information:
airport from/to (if flying)
last ACARS message received
last alert (if alert is set in the last 10 hours)
aircraft flying information
aircraft with an alert in the last 10 hours
Airport
Airports are shown as yellow dots on the world map. While moving over the airport icon, the tooltip will show up to give you more information:
airport ICAO
airport name
airport info
Aircraft route
The lime/grey lines are showing the aircraft route between the airports (shown as yellow dots). If the FROM/TO information is known - the aircraft route will connect both airports.