Some speakers don't display how many watts they are but instead they have the symbal for 'Ohms' I understand this means resistance, but how is it related to the performance of a speaker? cheers
By itself, the impedance (not resistance alone, though both are expressed as Ohms) of a speaker (whether a single driver or a complete speaker system) has no particular bearing on the speaker's performance. What is more important, though almost never stated in spec sheets, is the impedance swing, or curve, from highest to lowest points. This is far more an indicator of the potential problems an amplifier might face in dealing with a speaker than a single "nominal" impedance rating.
More to the point of your question, the idea that wattage in any fashion plays a role in the speaker's performance is essentially incorrect in every imaginable way. Speakers have no wattage and the role wattage alone plays in the performance of a speaker is substantially minimized by other factors concerned with reproduction of the signal. As with impedance, wattage is a combined formula of different values (put Ohm's Law into a search engine) where no single number can define exactly what is happening in a static specification.