There are several 'segment modes' which control the angle at which each segment of track or shape (including board outlines and areas) is laid down.
If you right-click when adding a track or shape, the Segment Mode submenu offers you several choices for how the segment pattern is laid down.
    Free : corners are placed where you click, with direct straight lines between each corner regardless of angle.
    Orthogonal : segments follow the shortest orthogonal path between the previous corner and the current position. Toggle between XY and YX using the Flip command which also works on the modes below. The dafault shortcut is <F>.
    Right-angle : similar to Orthogonal, but when you place a corner both segments are laid down.
    Mitre : creates two segments, one orthogonal and one angled at 45 degrees. These alternate as each corner is laid down.
    Fillet : similar to Mitre, but a 90-degree rounded corner is used instead of a 45-degree mitre.

When editing any shape or track, the hotkey <M> is the default for cycling through all possible angle modes (including flipped versions).