Your sleeping bag should be lightweight, warm, comfortable, easy to fold, with a hood or other way to cover your head and usually appropriate to the climate where you will camp.
The heat of a sleeping bag is provided by the insulation material that entraps the air. This heat will depend particularly on the type and amount of its insulation fill, the thickness (softness) of the filling, the size of the bag, style and method of construction.
How they are classified-Sleeping bags are usually classified into recommended use classes: for summer, three seasons or winter. Manufacturers give their bags comfort categories, designed to indicate the lowest temperature at which the bag will be warm and comfortable. Bag sorting methods vary from manufacturer to manufacturer, so these ratings can only give a general idea of how the bag will perform. The bags are usually classified by their comfortable minimum temperature, which can be between 5 and -30 degrees Celsius.
How close a category is to you will depend on whether you are inside a tent, clothing you are wearing, soil insulation, body size, metabolism, and calorie consumption. Look for a class of sleeping bags that matches your uses. If you put the backpack on the back to walk mainly in the summer, a bag indicated not too hot.