Many commonly used functions can be accessed from the toolbar for ease of access, and they can also be found in the various program menus.

Open Scene Opens a scene in the current window. If a scene is already open, it will be closed (following a prompt).
Close Scene Closes the current scene, terminating the current render (following a prompt). Generally you'll want to save your rendered image before closing the scene.
Render Start rendering the current scene. Upon pressing this button, the scene will be built and then rendering will start.
Pause Render Pauses or resumes the current render, freeing up your computer's CPU for other tasks. You can resume the render at any stage. If you need to run a short, processor-intensive task while rendering, pausing and then resuming the render is effective (Indigo gives itself below normal priority by default, so this is normally not a problem).
Stop Render This stops the current render, freeing the used CPU and memory resources for the currently rendering scene. You can still tone-map and save the rendered image after the render has been stopped, but unlike with Pause, you can't easily resume rendering (see Resuming a render if you need to stop and later resume a render).
Update Image Indigo only updates the displayed image occasionally, to avoid wasting processing power on image updates which show little visible difference. However, you can at any time press the Update Image button (or the F5 shortcut key) to force an image update.
Save Image Once a rendered image is displayed, you can click Save Image to save it to disk in a number of standard formats such as PNG or JPEG, as well as the special Indigo Image (IGI) format for resuming the render later; the PNG format is recommended since it doesn't degrade the image quality as JPEG does.
Network Rendering Enables network rendering. See the Network Rendering section for more information.
Options Opens the options dialog. See the Options Dialog section for more information.
Licensing Use this button to open the Licensing window. You can use this to buy an Indigo licence, immediately removing the restrictions of the free version. See the Licensing section for more information.
Pick Material This tool allows you to select a material by clicking on an object in the image view, whose currently applied material appears in the property editor.
Assign Material This tool allows you to apply the currently selected material (in the scene view) to an object in the image view by clicking on it.
Add Material Creates a new material in the scene view.
Add Medium Creates a new medium in the scene view.
Upload Material When editing a material, it uploads the material applied to the preview object together with the currently rendered preview image. The preview must be sufficiently clean (at least 200 samples per pixel) otherwise the upload will not succeed.
Online Material Database Accesses the Indigo online material database, allowing you to download materials into your scene and the local material database.