Base training consists of moderate effort training over the course of weeks and months. When done properly this period of training establishes good form and strengthens the body laying the foundation for more intense, longer duration, injury free activity later. Base training means different things to different people based on their sport specific fitness and experience. Developing a base for a sport we are new to will be different than the base period going into another training season for a sport with which we have lifelong experience.
Proper form is very important in the base period, more so if we are new to a sport. The base period is a time when focus needs to be placed on muscle memory and nervous system training. Maintaining the proper form is the concern which trumps all others - training with bad form will reinforce bad habits guaranteeing less than peak performance and possibly leading to injury. For this reason duration and intensity is limited by the ability to maintain form.
If when training competitively we discover a deficiency in our form, we have to abandon advanced training and return to the base period. For runners this could be the transition from heel striking to midsole. Recently for my swim it meant giving up thrashing around in an attempt to do laps to doing the drills in the Total Immersion perpetual motion freestyle DVDs. I find swimming and mixed martial arts such as Muay Thai and Jiu Jitsu require more drilling and careful thought than less mindful running and cycling.
The base period for sports with which we have lifelong experience is more about strength than form, as form should be mastered already. This period will involve strength training in some form. For runners and cyclists this does not have to be limited to weights and can include hill repeats, tempo work, sprints, etc… as long as the total activity remains moderate.
Successful completion of the base period of training will yield some 'base' level of stamina and endurance. From this 'base' the next phase of training, the 'build' can add duration and some intensity. Some inherent overlap exists between the base and the build phases. When done properly all phases of training should transition gradually, too much too soon is a recipe for injury and burnout.