Silicone injections are exactly what they sound like — injections of silicone into the body, and in this context generally (but not exclusively) to the male genitalia. The potential changes are, for lack of a better word, "monstrous." As far as male genital injections, circumference increases of as much as three hundred percent are definitely not unheard of. Done slowly, the level of sculpting that can be achieved is almost limitless.
In general, this procedure is not reversible, at least without extensive invasive and damaging surgery.
Silicone has been injected successfully in most parts of the body, including the penile shaft (under the skin, not in the cavernosum or the glans), the scrotum (but not in the testicles), legs, chest, arms, eyebrows, and so on. In theory, a skilled doctor could resculpt an area with enough talent and forethought. It is essential that the practitioner be well trained and experienced.
The more you get the more noticeable it will get. Sometimes patients have gone overboard and went so far as making sexual penetration next to impossible. While an aesthetically pleasing amount of extra penile width is a great thing, understand that the more silicone that you receive, the more your genitalia will look like a football.
Over repeated sessions the tissue is bulked up. Silicone injections are usually done with a special hypodermic syringe that has an additional feeder system allowing the silicone to be injected into the tissue at a certain cc per square inch ratio, while still being able to "shut off" the syringe and draw more silicone without having to remove the hypodermic tip from the body. It is not as easy as simply injecting silicone using a normal syringe. Many unscrupulous practitioners use far more primitive methods—often with disastrous results.
Longterm Health Issues
While most people who've had silicone injection work done have had no difficulties, severe reactions are possible from poor application, low grade materials, and even silicone allergies. This is made worse because the procedure is not generally legal in the US. The FDA warns that they have not approved "the marketing of liquid silicone for injection for any cosmetic purpose, including the treatment of facial defects or wrinkles, or enlarging the breasts. The adverse effects of liquid silicone injections have included movement of the silicone to other parts of the body, inflammation and discoloration of surrounding tissues, and the formation of granulomas (nodules of granulated, inflamed tissue)."
Silicone injection is illegal in the US, but is legal in some countries in Central and South America and many people travel to have the work done there. Underground doctors servicing those communities also exist in Canada and the US and can often be found through esthetics salons in ethnic communities (often immigrant doctors find it difficult due to prejudicial local laws to get their licenses and are forced to work in the underground plastic surgery market). Male genital enhancement groups and better drag queens often have domestic contacts for these procedures as well, both from doctors and non-medical practitioners — "silicone parties" where men gather to have work done are not unheard of in New York City for example.
Fat relocation is a fairly common procedure for male genital enlargement — check local urologists and plastic surgeons for availability in your area. Silicone injections are permanent, while fat injections may be reabsorbed to some extent.