Basically, they're titanium alloys that can be heat-treated to alter their properties, and are sort of the titanium version of tool steels and other highly heat-treatable iron alloys.
The term "beta titanium" refers to titanium that is alloyed with a good amount of other elements that are beta-phase stabilizers. The normal "alpha phase" crystalline structure of titanium at room temp is HCP (close-packed hexagonal). The beta phase of titanium is body-centered cubic, and with enough beta stabilizing elements, this crystal growth phase of the metal, which is brought about by heating the metal similarly to heat-treating steel, can be retained at room temperature after a rapid quench. This way, titanium can be heat-treated into a tougher, harder, stronger condition than alpha-state titanium, and can even produce several forms of hard titanium "martensite."
Grade 5 titanium is called an alpha-beta alloy, and it is sort of between the two. There are also near-alpha and near-beta alloys, which are just riding the edge between the two. Because a beta ti alloy can can assume either state permanently it's possible to create the more wild states of the metal through heat-treatment.
So I suppose a good comparison would be to say that tool steels could be considered "beta iron alloys!"
