Unalloyed titanium grades are often referred to by their tensile strengths, as in 35A, 50A, 65A, 75A, 100A ...
The difference has to do with sub-percentage traces of oxygen and iron in the metal. The more of these dopants (inclusions, contaminants) there is, the harder the metal becomes.

Links to more definition than anyone needs about these elements
Ti = Titanium
Al = Aluminum
V = Vanadium
Sn = Tin
Mo = Molybdenum
Zr = Zirconium
Pd = Palladium
Ni = Nickel
Cr = Chromium
Si = Silicon
Nb = Niobium
Ru = Ruthenium

ELI = Extra Low Interstitial

Flash animated version of Tom Lehrer's "The Elements" Just for fun

MrTitanium's List of
the Grades of Titanium

ASTM
Grade
Alloy
Composition
Min.
Tensile
(KSI)
Min.
Yield
(KSI)
Modulus of
Elasticity
(PSI-106)
1 Unalloyed Ti ("Pure") 35A 35 25 14.9
2 Unalloyed Ti ("Pure") 50A 50 40 14.9
3 Unalloyed Ti ("Pure") 65A 65 55 14.9
4 Unalloyed Ti ("Pure") 80A 80 70 15
5 Ti-6Al-4V 130 120 16.4
6 Ti-5Al-2.5Sn 120 115 16
7 Ti-0.15Pd 50 40 14.9
9 Ti-3Al-2.5V 90 70 13.1
10 Ti-11.5Mo-6Zr-4.5Sn 100 90 14.9
11 Ti-0.15Pd 35 25 14.9
12 Ti-0.3-Mo-0.8Ni 70 50 14.9
13 Ti-0.5Ni-0.05Ru 40 25 14.9
14 Ti-0.5Ni-0.05Ru 60 40 14.9
15 Ti-0.5Ni-0.05Ru 70 55 14.9
16 Ti-0.05Pd 50 40 14.9
17 Ti-0.05Pd 35 25 14.9
18 Ti-3Al-2.5V-0.05Pd 90 70 15.3
19 Ti-3Al-8V-6Cr-4Zr-4Mo 115 110 14.9
20 Ti-3Al-8V-6Cr-4Zr-4Mo-0.05Pd 115 110 14.9
21 Ti-15Mo-2.7Nb-3Al-0.25Si 115 110 14.9
23 Ti-6Al-4V ELI 115 110 16.3
24 Ti-6Al-4V-0.05Pd 130 120 16.4
25 Ti-6Al-4V-0.5Ni-0.05Pd 130 120 16.4
26 Ti-0.1Ru 50 40 14.9
27 Ti-0.1Ru 35 25 14.9
28 Ti-3Al-2.5V-0.1Ru 90 70 13.1
29 Ti-6Al-4V-0.1Ru 120 110 16.3

All of my artwork is made from Grade #1