Testosterone esters were synthesized for the first time in 1936, and were found to have greatly improved potency relative to testosterone.  Among the esters synthesized, testosterone propionate was the most potent, and for this reason, was selected for further development, subsequently being marketed.  Testosterone propionate was introduced in 1937 by Schering AG in Germany under the brand name Testoviron.  It was the first ester of testosterone to be introduced,  and was the major form of testosterone used medically before 1960.  In the 1950s, longer-acting testosterone esters like testosterone enanthate and testosterone cypionate were introduced and superseded testosterone propionate.  Although rarely used nowadays due to its short duration,  testosterone propionate remains medically available. 
Because the ultimate goal of a steroid cycle is to increase strength and muscle size, the associated spike in estrogen which accompanies steroids such as Testosterone is considered undesirable. In order to disassociate the two effects, two classes of drug are used. Medications such as Nolvadex or Clomid target the estrogen receptors. They make it more difficult for the estrogen to exert it’s influence within the body thus allowing the testosterone to act more freely. The second class is aromatase inhibitors such as Femara. They target the aromatase enzyme itself in order to prevent the production of estrogen in the first place. Sometimes, it’s not always clear which option you should go with or even what the differences are between the two. Lets clear that up a little.
Injectable steroids are injected into muscle tissue, not into the veins. They are slowly released from the muscles into the rest of the body, and may be detectable for months after last use. Injectable steroids can be oil-based or water-based. Injectable anabolic steroids which are oil-based have longer half-life than water-based steroids. Both steroid types have much longer half-lives than oral anabolic steroids. And this is proving to be a drawback for injectables as they have high probability of being detected in drug screening since their clearance times tend to be longer than orals. Athletes resolve this problem by using injectable testosterone early in the cycle then switch to orals when approaching the end of the cycle and drug testing is imminent.