The RMS value can be thought of as the "power content" - generally, the heating effect. If you have a 1kW electric fire, and apply 110V ac (RMS) to it, it will produce that amount of heat - the same as 110V DC. However, the actual peak voltage is in the region of 155V, but it only stays that high for a moment before it goes back down. Think of the RMS as the average figure between zero and 155.
With your speakers, 250W RMS is the maximum continuous power they will carry. Peak power figures with audio equipment are usually pretty meaningless, but a rough idea is how much you can get with a single surge - a cymbal clash, for example.
Use the speakers' RMS figure to match them to your amplifier (about 200-300W of amp), and ignore anything that's not RMS.