The tools are (from top-to-bottom):

The Select and Pan Tool switches between a (default) select mode for manipulating objects and an alternate
pan mode for quickly scrolling around the table.
The Layers Tool (GM Only) switches between the four layers on the tabletop:
- The Map & Background layer
- The Objects & Tokens layer (Note that players are always on the Objects/Tokens layer, and cannot switch between layers.)
- The GM Info Overlay layer (which is only visible to the GM)
- The Dynamic Lighting layer (Plus/Pro Feature - GM Only).
The Drawing Tools let you draw free-hand as well as shapes on the tabletop that everyone can see. You can also create text.
The FX Tool (Pro Feature - GM Only) lets you add special effects to the tabletop such as Burn, Explode, Dragon's Breath, and Fireball.
The Zoom Tool lets you zoom in or out on the tabletop.
The Ruler Tool lets you click and drag to measure distances on the table. While you measure, other players will see your measurement line so you can discuss what you're measuring.
The Place Light Tool (GM Only) is a tool available to all plus and pro members, which allows you to place a token emitting 20 feet Bright Light and 20 feet Low Light on whatever layer you have selected.
The Fog of War Tool (GM Only) lets you reveal areas of the map gradually so you can hide things you don't want your players to see just yet.
The Turn Tracker Tool (GM Only) lets you open up a window to keep track of turn order during encounters. Only the GM can see the button to open the window, but once it's open all players can see the window and the order of the tokens.
The Dice Rolling GUI is a quick and easy way to roll dice in Roll20, rather than needing to manually type in a formula. It's great for beginning players.
The Help Tool is a quick way to open a window that shows the Help site (which you're reading right now!), report bugs, and consult the list of keyboard shortcuts.