Switzerland definitely experiences seasons -- winter can bring a fair amount of snow while summer can be sunny and sultry but the weather is rarely extreme.  Temperaures in summer can climb to the 30s°C (90s°F) or even mid 30s°C (mid 90s°F). The Italian region, which includes popular destinations like Locarno, Lugano, and Bellinzona, can be more mediterranean (as evidenced by the palm trees and colorful birds that you'll see).  The French region is a bit more temperate while the areas in the north tend to be about 10 degrees Fahrenheit cooler in general.  Early summer (June) or fall is probably the best time to experience Switzerland, as the weather is comfortable and there aren’t as many tourists as there are during the summer (or the winter in the ski regions). September can be quite rainy, but October offers sunny weather with temperatures on their 20s°C (70s°F), which is perfect for hiking in the Alps even in high altitude.

Depending on when and where you go, the weather in Switzerland can vary greatly due to drastic altitude differences.  High in the Swiss mountains, the weather tends to be much cooler than expected so bring a warm jacket, extra layers, and pants even in the summer if you plan on visiting any of the major mountains.  Some areas, like Les Diablerets and Zermatt, are cold enough that they are open for skiing even in the summer (though skiing Zermatt is certainly not recommended for newbies).