Single line diagrams generally where all the conductors of a circuit are represented by a single line - so it shows what things are connected, but omits the detail of how they are connected. If you like it shows only cables, not individual wires.
As for other symbols on those diagrams, that rather depends on the context - a simple rectangle for example could represent anything from a joint box to a building - in some cases common electronic symbols (e.g. batteries or inverters) could appear - but often you need to refer to labelling or a key or the surrounding context to work out what represents exactly what.
Single line diagrams generally where all the conductors of a circuit are represented by a single line - so it shows what things are connected, but omits the detail of how they are connected. If you like it shows only cables, not individual wires.
As for other symbols on those diagrams, that rather depends on the context - a simple rectangle for example could represent anything from a joint box to a building - in some cases common electronic symbols (e.g. batteries or inverters) could appear - but often you need to refer to labelling or a key or the surrounding context to work out what represents exactly what.
