Concrete contains cement. Concrete is a combination of cement as a binding agent and aggregates such as sand and stone.
If you have a small job then it will be more cost effective to use bagged product. If you have a large job then it will be easier to get a truck in to deliver a large load for a big job.
Concrete is made from cement, water, and aggregate.
The colder the weather the longer it will take to cure. If the weather is extremely cold then the water in the concrete can freeze and will cause problems for your pour.