These are alloys of Titanium
with other elements added to stabilise one or other of the two possible crystallographic phases that the element forms and hence affect the properties of the resulting material.
Titanium has two of these phases, one is Alpha and the other denoted as Beta. These are HCP and BCC respectively.
Adding elements with electron:atom ratios of less than 4 stabilise the alpha phase, whereas those with a ratio of more than 4 stabilise the Beta phase. Those with a ratio of four are neutral.
There are three main classifications of Titanium alloys, Alpha Titanium Alloys, Alpha + Beta Titanium Alloys and Beta Titanium Alloys. These all have their individual uses, according to their properties.