Have you ever seen this icon next to your mouse cursor?
It can turn out to be the cause of a lot of frustration as SOLIDWORKS seemingly "locks up" when this icon appears preventing you from selecting parts of you model on screen. The reason for this is that you have inadvertently activated a Selection Filter. Once you know what they are and how to use them hopefully your frustration may ease up and they can turn out to be very handy features.
Take for example selecting edges to add fillets to- have you ever accidentally clicked a face instead and found the fillet preview goes crazy? How about in an assembly picking the wrong piece of geometry to add mates with. This is where selection filters come into their own. You can activate filters that only allow you to pick certain aspects of the model or drawing meaning you'll pick the right things first time.
To take a look at the full list of available filters, press the F5 key and you will see a new toolbar appears at the bottom of your screen.
Looking across the commands you will see filters for dimensions- great for box selecting a drawing to hide/delete only dimensions, how about Filter Bodies when you need to add a mesh control to an entire part in a Simulation, they really do make things easier.
So now you know about the toolbar, you may be wondering why the filters appeared in the first place- well this is down to keyboard shortcuts- the Edge (E), Vertex (V) and Face (X) Filters have keystrokes assigned to them. Talking of keyboard shortcuts F6 is a good one on this topic as it clears all active filters.