These two words have different meanings depending on the context used. In physics "Tension" is a pulling-force exerted by a string, cable, chain, or similar solid object on another object. Again in physics, "strain" is the linear deformation due to tension. Understanding this, you can relate those meanings to other contexts. For example: Tension can develop between two people disagreeing. Strain occurs when tension is at its greatest... almost to the point where something gives. Thus in this case, as an example, so much strain could cause the two people go from a simple disagreement to fighting. I suppose you can look at it as too much "tension" causes "strain."