For most of the year, New Orleans is hot and muggy. In the summer, the temperature is often well into the 90s and it can be difficult to escape the humidity outside of retreating to your hotel's swimming pool, if it's got one. If you don't mind the heat and humidity, summer is a good time to grab some hotel bargains.
There is a short winter in New Orleans, although it doesn't get particularly cold. Some days in February you may find yourself walking around in shirt sleeves. On the other hand, one of the city's best known ghost stories involves a woman who froze to death outside one January night. So don't show up in winter time dressed for the tropics.
The weather is very nice in the spring in New Orleans, although Mardi Gras is something to factor into your travel plans. Another factor to consider is the hurricane season that runs through late summer and early fall. This is one of those parts of the country where more often than not, there is warm or hot sunshine. But when it rains, it can rain hard. Before the levees failed in 2005, even a heavy pounding day-long rainstorm would be enough to fill some parts of the city with knee-deep water.