Learn Finnish: Sanasto jaetaan aiheittain. - The vocabulary is divided by topic.

It comes from the noun aihe (topic). The suffix -ittain (or -ittäin) means "by [something]" or "group by group". To add it, you use the plural stem of the word. For aihe, the plural stem is aihei-, so adding the suffix gives us aiheittain (by topic).

Why is the verb jaetaan in the passive voice?

Jaetaan is the present passive of jakaa (to divide). The passive is used here because the sentence doesn't specify who is dividing the vocabulary; it is a general statement that the vocabulary "is divided".

Why is there no 'k' in jaetaan if the dictionary form is jakaa?

The verb jakaa undergoes consonant gradation. When forming the passive, the strong 'k' drops out. This leaves us with the weak stem jaa-, and the passive ending -etaan is added to make jaetaan.

Why doesn't the object sanasto have an object marker (like an -n ending)?

In Finnish passive sentences, a "total object" (where the action applies to the whole thing) takes the nominative case rather than the genitive-accusative. Because the entire vocabulary is being divided, sanasto appears in its basic, uninflected form.