There are three types of Muscular Contractions: Concentric, Eccentric and Isometric. A Contraction is anytime a muscle generates "force". The force that would be applied by the contracting muscle on an object would be the muscle tension, the force of the resistance or weight on the muscle itself would be the load.
When a muscle shortens against a load or when you raise the weight would be a concentric contraction, this could be referred to as a positive phase of a lift or movement. The muscular tension would be greater than the load and a shortening of that muscle would be achieved. An example of this would be when peforming the biceps curl the concentirc phase would be when you are raising the weight up towards your shoulders.
When the muscle lengthens against a load or when you lower the weight would be a eccentric contraction, this would be referred to as the negative phase of the lift or movement. The muscular tension would be less than the external load and a lengthening of the muscle attained. An example of this would be when performing the Biceps curl the eccentric phase would be when you are lowering the weight towards the ground.
When the muscles contractile component shortens and the elastic component lengthens at the same time producing no change in the overall length of the contracting muscle would be an isometric contraction. The muscular tension would be equal to the external load and no movement would be achieved but metabolic work is accomplished. This would be as if you exert tension/force against a solid/immovable object or hold a weight in a static position. An example of this would be when holding weight midway through a biceps curl in a static position (1/2 through the lift with NO movement of the weight) for a period of time.