What's the relationship between Thanos and Ronan?

Question

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?

Answer 1

They were military allies.

Ronan agreed to help Thanos collect the Orb, in exchange for help destroying Xandar. Later, he learns that the Orb is an infinity stone and goes back on his deal.

Here's a nice summary:

Ronan first appears in the 2014 film Guardians of the Galaxy. Depicted as an avid Kree fanatic whose family was killed in the Kree-Nova War, he refuses to heed his Empire's peace treaty with the Nova Corps of Xandar and embarks on a genocidal campaign against all Xandarians. Ronan agrees to recover a mystical orb for Thanos in exchange for help in destroying Xandar, only to renege on their deal when he learns the orb contains an Infinity Stone that he can use to annihilate Xandar himself. Ronan then decimates the Nova Corps' fleet, but the Guardians of the Galaxy ultimately manage to take the Stone back and use it to obliterate him.

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_{Ronan the Accuser - Wikipedia}

So, they were using each other to achieve their goals, and then fell apart.