What's the difference between killstreak, specialized killstreak, and professional killstreak weapons?
Solution 1:
This is taken directly from the wiki:
Standard
...simply add[s] a HUD kill counter
Specialized
...adds a kill counter as well as a colored sheen that intensifies as streaks increase
Professional
...in addition to adding a kill counter and colored sheen, also causes the weapon to add a particle effect to the user's eyes (similar to the Eyelander) that intensifies with kills
Source: 1 | 2