Querido or querida is the right word to use when you speak to your wife or your husband, a person with whom you keep a very close relationship. A relationship of love, for instance.
On the other hand this word is very much used in the headings of the letters addressed to somebody with whom you keep a relationship of trust and/or friendship. It usually goes together with the name of the person you are writing to, such as: Querido Juan, Querida Juana, Querida amiga, etc.
Estimado indicates a relationship of friendship but more distant and dettached than querido.
The director of your bank will write you a letter and very possibly the heading will be: Estimado señor Perryinjax.....
Spanish language has two treatments that indicate the relationship existing between persons.
The first one is Usted (Translated to English like "You"). This is a treatment of respect. We use it to speak to our boss, the director of our bank, or a person whom we simply do not know. There is a distance with this "Usted" word, respect and deference are implied.
The second one is Tú (with this little thing on top of the letter u). This is translated to English like "You", English language has long ago forgotten the word "thou" that is exclusively to be used to talk to God.
Tú implies a relationship of friendship and trust, it is the word you use with your wife or husband, with your children, friends, etc.
Now, these two words have, each one, a possessive pronoun.
The possessive pronoun for Usted is "Su". It means your, yours.
Señor García su coche necesita reparación.
Mr. García your car needs a repair.
This implies that you are treating Mr. García with great respect and defference - you call him "Señor", you use the possessive "su" . Therefore the word "Usted" is implied here even if we do not see it.
The possessive pronoun for Tú is "tu" (no silly thing on top of the letter "u"). It means your, yours.
Pepe, tu coche necesita limpieza. Si no lo limpias tú tendré que limpiarlo yo.
Pepe, your car needs to be cleaned (a cleaning). If you do not clean it I will have to clean it myself.
We can see that we have a friend talking to a friend here. Compare Pepe (the nickname for José) with "Señor García" above.
Tu coche................ Your car
Si no lo limpias tú------------------------------- If you do not clean it...........
And that is all. I hope that your thirst for knowledge has been properly quenched.
Su seguro servidor,
Trommel