Voltage across a resistor
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simulate this circuit &ndash; Schematic created using CircuitLab

Voltage is always measured between two points, so when we quote the voltage of a single point, it's always understood that we are measuring the difference to some reference point.

I've taken the liberty of adding the ground or '0V' symbol. You've told us that 'A' is at 5v, which implies that you're taking the negative terminal of your power supply as your 0V reference.

No current flows through R1, so the voltage across it is zero. This means that B is also at 5v.

Voltage is a potential. A good analogy is 'vertical height' in earth's gravity field, as far as energy goes. 

In fact, it's such a well ingrained analogy that you'll notice I drew the diagram with height up the page to represent voltage. Most electronic engineers will do this, as it makes any schematic easier to understand.