As we can see from the animated clipart, Gait is a complex cyclical activity.  we can see that at one point of time one limb is swinging , while the other one is in contact with the ground. This is the starting point for the understanding of Gait cycle:  stance phase and swing phase. 

A full gait cycle is described as the undertaking of both stance and swing phases by one limb. The aim of gait is to accomplish three key functions: weight acceptance, single limb support, limb advancement. The stance phase occupies 60% of the gait cycle. The phases are divided into different events, Perry's description is helpful to study pathological events of gait. Perry describes five events in the stance phase: initial contact, loading response, mid stance, terminal stance, pre-swing. The events in swing phase are: initial swing, mid-swing, terminal swing. The propulsive force in walking is the ground reaction force. 

Ground reaction force , being either anterior or posterior to the joint , creates flexion or extension momentum and necessitates appropriate muscle activation. 

Weight acceptance: The task here is to prepare an unstable swinging limb to accept rapid transfer of body weight in a stable platform and thus to preserve the aim of gait: progression. It has two phases:                                                         

Initial contact : when foot touches the ground .Hip is flexed, knee is extended, ankle is dorsiflexed to neutral, floor contact is made with heel. Ground reaction force is anterior to the joints except ankle where it is posterior and creates plantar-flexion moment.

  Moment due to GRF and muscle activity to couneract moment

Loading response: this phase begins with full floor contact and involves a brief period of double limb support. This phase ends with opposite foot lifted for swing heralding the beginning of single limb support. As the body weight is transferred onto the forward limb, knee is flexed for shock absorption. Knee flexion is brought on by GRF vector going behind the joint.

Single limb support: In this phase the single supporting limb supports body weight in both sagittal and coronal planes, while maintaining progression. 

Mid stance: first half of single limb support. It finishes when body weight is aligned over the forefoot. 

Terminal stance: In this phase, body weight moves ahead of the forefoot. 

Limb advancement: Limb is prepared from end of stance phase to advance and forward limb for progression. There is no GRF in swing phase. Limb progresses due to inertial force and muscle activation is required to control it.

Pre-swing: this is the final double stance interval. During this stage the supporting limb off-loads weight and prepares to swing.

Initial swing: The objective of this stage is to clear foot off the floor and advance it. It ends when swinging foot lies opposite the stance foot. It is achieved by hip amd knee and ankle flexion. 

Mid swing: In this phase foot is advanced and ends when tibia lies vertical.

Terminal swing: During this phase leg moves ahead of the thigh and ends when foot strikes the ground.