A key component of teaching is effective classroom management. This is the set of steps you follow to ensure that your students pay attention, don't distract each other and generally stay on task. This is different from discipline, which is just one part of classroom management. Where discipline describes the consequences you give students for not following the rules, classroom management describes a more general set of procedures, most of which are aimed at avoiding problems rather than responding to them.
The classroom setup is an example of classroom management that is not discipline. After a few weeks of teaching, it becomes fairly clear which students should not be sitting near one another, as certain friends (and enemies) will distract one another and the children around them for the entire lesson. Discipline would be punishing these children every time they disrupt the class; classroom management is moving them somewhere else to keep the disruption from happening in the first place.
Another example of the difference between discipline and classroom management is the classroom rules. Classroom management is when you make the rules clear to the children, either through discussion or by teaching through another method. Posting these rules in a prominent place is another way to help manage your classroom -- by making the rules clear to children and making them visible, you make it less likely that the rules will be violated.
Discipline is how you respond to violations of these rules. This makes rules an excellent way to highlight these differences -- classroom management is the front end of the rules and discipline is the back end.
Classroom management is also a matter of keeping students occupied, either in a lesson, discussion or activity. When children have something to focus on, they are less likely to create their own stimuli by "zoning out" or misbehaving. So, particularly for younger years, it is strongly recommended that teachers overplan their lessons in order to always give the children something to do.
Discipline is a matter of dishing out consequences when students go off task, whether the lesson is well-planned or not. In general, the more thoroughly occupied students are, the less discipline they will need.
A final example of a difference between classroom management and discipline is the general tone you set. You set a tone in classroom management by your confidence, the way you present yourself and how well you relate to students. If you do these well, your classroom will be well-managed because it will be clear to students who is in control.
Discipline also requires tone-setting. Once you've made the rules clear, you need to follow through the minute someone violates them. This is basically setting an example. It's often not enough to simply have rules; rather, you need to let students know you're serious. This concept and that mentioned above are examples of setting a tone in which the teacher is in control and creating a positive learning environment.