For TvZ and PvZ, but with greater emphasis on TvZ since I am a Terran user.
"You may advance and be absolutely irresistible, if you make for the enemy's weak points; you may retire and be safe from pursuit if your movements are more rapid than those of the enemy."
"If we wish to fight, the enemy can be forced to an engagement even though he be sheltered behind a high rampart and a deep ditch. All we need do is attack some other place that he will be obliged to relieve."
"The spot where we intend to fight must not be made known; for then the enemy will have to prepare against a possible attack at several different points;"
"Attack is the secret of defense; defense is the planning of an attack."
Good Zerg players will use their Overlord and Zergling to scout any units moving toward their base. In a nutshell, this is how Zerg players operate in their mind.
(1) No units spotted => Maximum Drone production while keeping minimal defense force for possible harass
(2) Units spotted by scouts => Maximum fighting unit production according to the information you have for appropriate counters
Hiding your forces from Zerg scouts will force Zerg to either A) make incorrect decisions and do (1) and (2) out of guts, or; B) sacrifice something(eg. couple lings, changeling, or ovie) to reveal more information. It is often impossible for Terran to hide everything from good Zerg players.
Therefore it is imperative to purposely give out information (that you want to give) to Zerg in order to achieve this: to have them do (2) while they should have been doing (1). How? By being aggressive in a game, but not over-committing yourself!
This strategy has been used for more than a decade by BW players; Terran often sent a squad of Marines and Medics and force Zerg to sacrifice Drones to build Sunkens, and sacrifice Drone production to make Zerglings.
In SC2, Terran has ample tools of such aggressiveness shown in BW, but without dangers of over-committing your army.
3. Marauder (for Slow)
4. Medivac (for loading/unloading)
6. Salvageable Bunker
Even though more methods are available for Terran players to be aggressive, Zerg also has two great tools themselves to maximize efficiency and effectiveness of (1) and (2) - Queen's Spawn Larvae and Creep Tumor abilities! This enabled Zerg to do both (1) and (2) twice as fast while detecting/defending incoming squad extremely well.
---More to be added---