"Testosterone booster" is a marketing phrase. In reality, your body has a maximum set point (like you a/c or heater) that shuts off once you reach a certain preset level. The challenge for males is that our natural testosterone level peaks in our early to mid 20's and then starts a long slow decline into our 70's with a lightly increased dip somewhere in our 40's. This can be substituted with testosterone (I recommend the once a week injection over the daily cremes/gels.
All that said, you can have depressed testosterone level, below where it should be; mainly caused by poor diet and/or lack of exercise, which in turn promotes obesity, diabetes and cardiovascular disease, all of which depress the test levels and can contribute to erectile dysfunction issues. SO, as I routinely tell people: outside of protein (find one low in sugar) or creatine, most workout boosting supplements drain your wallet and that's about it. The best testosterone boosters are eating well and exercising, and there are some specific exercises, like barbell squats and dead lifts, which have been found to cause a boost in the body's production of testosterone because of the overall demand they place on the muscles and the entire body. I will offer the same caution against testosterone "precursors" as the liver readily metabolizes and alcohol group on the 17th carbon of the testosterone molecule. As such, oral testosterones (like Dianabol (methyltestosterone)) are very hard on the liver and have significantly more side effects, like water retention and corresponding elevated blood pressure, due to the alteration in the molecule to protect the alcohol group from first pass metabolism so it can get out into the body and have an effect. This is is why injectable (or, if you really must, topical cremes.gels) are preferable, as they do not require any molecular alteration, and this lack all those side effects found in the oral dose forms.
So, anything that you swallow that claims it will do things for your testosterone levels is a marketing gimmick. Put your time in on your dietary choices (get rid of sodas (yes, even the diet ones) and have maybe one a month as treat--I consume between nine and fourteen single can sodas a year while many Americans go through that many in a couple days) and avoid fast food restaurants unless you are getting a grilled chicken sandwich and a baked potato (just butter and maybe cheese) and put your time in in the gym. If you are over 40 and feel like your testosterone levels may be low, see your physician to get your levels checked and see if an actual testosterone prescription is needed to supplement your lower levels.