Options

This dialog allows you to select some basic options for QGIS. There are eight tabs:

General
Rendering & SVG
Map tools
Overlay
Digitizing
CRS
Locale
Network & Proxy

General

On this tab you can set general options for project behaviour on opening/saving, default map appearance, your favourite icon theme and many others.

Rendering & SVG

Within this tab you can set general rendering options for map layers.
By default, QGIS renders all visible layers whenever the map canvas must be refreshed. The events that trigger a refresh of the map canvas include:

Adding a layer
Panning or zooming
Resizing the QGIS window
Changing the visibility of a layer or layers

QGIS allows you to control the rendering process in a number of ways:

You can set an option to always load new layers without drawing them.
This means the layer will be added to the map, but its visibility checkbox in the legend will be unchecked by default. To set this option, uncheck the By default new layers added to the map should be displayed checkbox.

You can set an option to update the map display as features are drawn.
By default, QGIS does not display any features for a layer until the entire layer has been rendered. To update the display as features are read from the datastore, set the Number of features to draw before updating the display to an appropriate value. Setting a value of 0 disables update during drawing (this is the default). Setting a value too low will result in poor performance as the map canvas is continually updated during the reading of the features. A suggested value to start with is 500.

Map tools

The Mode setting determines which layers will be shown by the Identify tool. By switching to Top down instead of Current layer attributes for all identifiable layers will be shown with the Identify tool.

Overlay

Define placement algorithm for labels. Choose between central point (fastest), chain (fast), popmusic tabu chain (slow), popmusic tabu (slow) and popmusic chain (very slow).

Digitizing

In the Digitizing tab you can define a general, project wide snapping tolerance.

You can select between to vertex, to segment or to vertex and segment as default snap mode. You can also define a default snapping tolerance and a search radius for vertex edits. The tolerance can be set either in map units or in pixels. The advantage of choosing pixels, is that the snapping tolerance doesn't have to be changed after zoom operations.

A layer based snapping tolerance can be defined by choosing Settings (or File) > Project Properties.... In the General tab, section Digitize you can click on Snapping options... to enable and adjust snapping mode and tolerance on a layer basis.

CRS

QGIS no longer sets the map CRS to the coordinate reference system of the first layer loaded. When you start a QGIS session with layers that do not have a CRS, you need to check and define the CRS definition for these layers. This can be done globally on this tab.

The global default CRS proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs comes predefined in QGIS but can of course be changed, and the new definition will be saved for subsequent QGIS sessions.

Locale

Informs you about active system locale. To change it tick Override system locale and restart QGIS.

Network & Proxy

Tick Use proxy for web access to define host, port, user, and password.