From the More Beer website, referencing the Siebel Institute (though I can't vouch for its veracity):
"Yeast growth can be divided into five stages.
Lag phase occurs during the first few hours after inoculation. Although no signs of fermentation or growth are apparent, the yeast are busy becoming acclimated to their new environment. All the enzymes and other metabolic machinery necessary to ferment wort into beer are being synthesized. All the oxygen you introduced into the cooled wort is taken up by the yeast within the first 20 min and is being stored (as sterols and unsaturated fatty acids) for later use.
During the accelerating growth phase, yeast cells start to divide rapidly.
The actual number of yeast cells increases during the logarithmic phase. During this time the number of yeast cells may increase as much as 1000-fold (or 3.0 logs) within 24 h.
As the oxygen and nutrients are depleted, the yeast enter a phase of decelerating growth (approximately 12 h) and ultimately reach a stationary phase.
During the stationary phase, yeast growth ceases.
Despite the rapid rate of yeast growth, a relatively large yeast starter or slurry of yeast is required for optimal beer production. Siebel Institute recommends one-sixth of the batch-size, one-tenth if you continuously aerate or agitate your starter. The scientific basis for this is currently unknown."
A friend (pro brewer) who attended and is Siebel credentialed used to always reference the lag and log phase as simply the preparation (for growth) and the growth phases...FWIW.