This needs paper and pencil, but it's an easy explanation (even if imperfect).
If you draw a graph of temperature vs time, you will see a wave pattern emerge over a few years. Global warming implies that there is more energy in the atmosphere, so the amplitude of the wave will increase. This means summers get hotter and winters get colder.
The scientific debate is about how much of an amplitude change will result, and what the consequences of that will be (we don't know this yet).