This is Africa and the climate reflects it. But just as Namibia is filled with contrasting geography, equivalent climactic differences do apply depending on your location.
Partially covered by the Namib, one of the world's driest deserts, Namibia's climate is generally very dry and pleasant. The cold Benguela current keeps the coast cool, damp and free of rain for most of the year. Inland, all the rain falls in summer (November to April). January and February are hot, when daytime temperatures in the interior can exceed 40ºC (104ºF), but nights are usually cool. Winter nights can be fairly cold, but days are generally warm and quite nice.
The bottom line: Namibia is a year-round destination. Just pack accordingly.