There are four important conformations of cyclohexane; Chair, boat, half-chair and twist-boat.
The most stable is the chair conformation. In the chair conformation all the bond angles are at the optimal value of 109.5o and all the hydrogens are staggered. There are two different types of protons in the chair structure; axial (green below) and equatorial (blue). If you imagine the chair structure is a wheel then the axial protons would be parallel to the axle and the equatorial protons are around the "equator" of the ring.
cyclohexaneJMOLSCRIPT{select atomno=10, atomno=13, atomno=17, atomno=8, atomno=12, atomno=15; color atoms green; select atomno=14, atomno=16, atomno=18, atomno=7, atomno=9, atomno=11; color atoms blue;}
The boat cyclohexane conformation kind of looks like a boat, hence the name "boat". It is less stable than the chair conformation by about 6.5 kcal/mol. The instability comes from the steric interaction between the flagpole hydrogens (depicted in blue) and the torsional strain from the eclipsed hydrogens along the sides of the boat (green).
cyclohexane_boatJMOLSCRIPT{select atomno=11, atomno=17; color atoms blue;select atomno=13, atomno=14, atomno=15, atomno=16, atomno=8, atomno=9, atomno=7,atomno=10; color atoms green;}
The half-chair conformation of cyclohexane is a transition state between the chair and the twist boat conformations. It is destabilized by about 10.8 kcal/mol relative to the chair conformation.
The twist boat cyclohexane conformation is about 5.5 kcal/mol destabilized (i.e. higher in energy) than the chair conformation. Cyclohexane does not spend much time in this conformation. At room temperature there is only about 0.1% of the twist boat.