Below is a description of each part of the interface. The icons themselves change depending on the last option you chose from the list.

The Toolbar

From left to right:

Blue Icons:

Save Game, Load Game, About, Options, Screenshot, Quit Game
Mute, Play Music, Music Options

Green Icons:

Zoom In, Zoom Out, Map
Rotate Clockwise, Rotate Anti-Clockwise
View Options, Hide tracks, hide towns, display signs

Brown Icons:

Clear Area, Adjust Land, Adjust Water, Plant Trees
Build Railroad Track, Build Tram Track
Build One-way Roads, Build Roads, Build Rough Roads
Build Airports, Build Ship Ports
Build Trains, Buses, Trucks, Trams, Aircraft, Ships

Grey Icons:

View Trains, Buses, Trucks, Trams, Aircraft, Ships
View Stations, Rail Stations, Road Stations, Airports, Ship Ports
View Towns, View Industries