Paris generally has a relatively moderate climate. In winter, temperatures can dip below freezing, but they usually stay around 45 F/7 C. Often winter is very wet and gray, although the rare snowfall disappears as soon as it hits the ground.
Summer mornings tend to be cool (sometimes requiring a sweater), but afternoons get warm. A few summer days get hot (80-90 F/26-32 C), broken by rainfall in August. Every so often, summer brings a canicule (heat wave) so you will want to check that your hotel has air-conditioning; most do, but some smaller ones may not. Dress warmly in winter and take along a sweater in summer.
Rainfall is more frequent in the winter months, but it can rain in Paris at any time of year, so it's always a good idea to pack rain gear. The best time to visit is May-July, although the cool fall months can be lovely. Most Parisians go on vacation in August, so many shops and restaurants may be closed then. Services may be limited as well.