Horizontal power sharing means power is shared among different organs at the same level — legislature, executive, and judiciary. Each organ checks the others (system of checks and balances), so no single organ has unlimited power.
Vertical power sharing means power is divided among governments at different levels — Central, State, and Local governments. Higher and lower levels of government share power, as in India's federal system.
Source: Forms of Power-Sharing, Chapter 1
The examiner expects you to clearly contrast the two types using the key terms: same level (horizontal) and different levels (vertical). Give one example for each — legislature/executive/judiciary for horizontal; Central/State/Local for vertical. Avoid writing long explanations; two crisp points with examples are enough for 2 marks.