On the coast the weather is tourist-friendly pretty much all year round, although winter can bring cool and wet conditions in the north.
In the South,(for example Marrakech) the cooler months from October to April are popular among visitors. This time of year is pleasantly warm to hot (around 30°C) during the day and cool to cold (around 15°C) at night. Winter in the higher regions like the Atlas Mountains demands some serious insulation.
If you're heading into the hills, the ski season usually lasts from December to March. For most trekking trips you should book in the high season (June 15 to September 15) or you may find areas full.