Tribulus terrestris is a herb from Ayurveda that is mostly recommended for male health including virility and vitality, and specifically more catered towards cardiovascular and urogenital health. It is a common supplement for its libido enhancing properties and supposed testosterone boosting properties.
On the sexual side of things, tribulus does appear to be a relatively reliable and potent libido enhancer in rats and the lone human study assessing this has confirmed an increase in sexual well being and erectile function. While it is not exactly known how tribulus works, it is known to enhance androgen receptor density in the brain (muscle tissue not confirmed) which may enhance the libido enhancing properties of androgens. Limited evidence suggests that it is weak to non-effective in enhancing fertility.
A specific component, tribulosin, appears to be quite potently cardioprotective and is effective in the 1-10nM range. It has not yet been tested in living creatures, but remains a very promising option.
In animal research, the fruits of tribulus appears to protect the organs (mostly liver and kidneys) from oxidative damages at reasonably low dosages and also exert anti-stress effects; confirming the status of tribulus terrestris as an adaptogen.
Despite the above promise as an adaptogen and a libido enhancer, studies investigating tribulus in sports performance have all failed to find benefits. The herb seems to be a possibly healthy herb that enhances sexuality but with limited use for power output and testosterone enhancement (which it has repeatedly failed to do).