A rupture is the big Daddy, the Nebuchanezzer of the muscular strain.  Whereas a strain is a section of the muscular fibres (say a bicep) that have been torn or frayed, a rupture is a majority disconnection of the entire muscle from its tendinous connection.  Usually, the ‘rupture’ is actually occurring not within the muscle itself but in the tendon that attaches the muscle to its bony connections.  By definition, a rupture is ‘to burst, break through or split under pressure’.  In the case of a tendon a rupture is defined by the muscle being unable to perform it's action.  The rupture can be partial or complete, both requiring surgery to ‘re-attach’ the torn tendon.