Occluded fronts are part of our low pressure aka cyclonic weather systems and are usually a sign that they've reached maturity in their lifecycle. A front is the (unseen) transition between two masses of air with distinct differences in temperature and with our low pressure systems a cold front follows on behind a warm front.
The colder air mass travels more quickly than its warm predecessor and if it catches up with it, then the warm air is forced off the ground and over the colder air. It's at this time that an occluded front is said to have formed. The weather found along these can be quite variable and can be some of the more extreme weather associated with a low pressure system, such as thunderstorms and funnel clouds.
Each type of weather front is shown by a different coloured line and associated symbol on a weather map - see the example shown here. The way the symbols are pointing shows the direction of travel of the weather system. Occluded fronts are denoted by a purple line with alternating semi circles and triangles, so the next time you see these on a weather map and it's heading your way, watch out for stormy weather!
For more O's than you can shake a stick at, do have a look at the ABC Wednesday blog.