In the Marvel Cinematic Universe Thanos and Ronan are complete opposites.
Thanos is cold, calculated and determined, he never does anything without thinking/planning/preparing/strategizing for/of every possible option and outcome.
Ronan is driven by his obsession for death and destruction (Thanos himself said it to Gamora), he's a reckless, genocidal lunatic even when he doesn't have the power stone. I highly doubt he thinks before taking action, Yon-Rogg once tricked him into carrying out a ballistic missile strike on C53 (Earth) after telling him that some Skrull soldiers/spies were there over the phone.
Even when Ronan had the power stone, a racoon, a tree, a grey guy with daggers, a green woman, a demi-god (with no powers) and a band of mercenaries managed to steal it from him.
In comparison, the only way all of the Avengers (combined) were able to steal the stones from Thanos was by going back in time and taking them before he got to them.
Another comparison are the Celestials who, according to the collector:
used the stones to mow down entire civilizations like wheat in a field
Ronan needed help from conventional military forces to do the same, despite having the stone in his possession.
So, what's the relationship between Thanos and Ronan, what do they do for each other and why are they working together?