Kaiser and Tsar are the same man: Caesar.
After Julius Caesar, Roman rulers carried 'Caesar' as part of their title; from Diocletian onwards it marked the designated successor. Germanic peoples borrowed the word early, when the C was still pronounced like K, giving 'Kaiser'. Slavic languages derived 'Tsar' from it; Ivan IV crowned himself the first Russian tsar in 1547. Two languages, one family name.
Useful: When you hear a title, notice which power is claiming to be Rome's heir.
Source: en.wikipedia.org