Cyclic organic molecules typically exhibit a certain degree of ring strain that makes them less stable than their linear counterparts. Ring strain results from angle strain (deviation of bond angles from 109.5 degrees) in small rings and torsional strain. It is largest for 3 and 4 membered rings and tends to be small for medium-sized rings.
Cyclohexane is the only cycloalkane, which has no ring strain. This is because cyclohexane can adopt the chair conformation, which completely avoids eclipsing interactions. The chair contains axial and equatorial bond positions, which can be interchanged via a chair-chair interconversion. Such interconversions go through a less stable boat conformation, which contains significant amounts of eclipsing interactions. Substituent groups prefer to adopt the equatorial positions because there is less steric hindrance from 1,3-diaxial interactions. Some groups such as the tert-butyl group are so large that they altogether prevent the chair from flipping into other conformations once the groups are in the equatorial positions.