ser vs parecer
Ser states what something is. Parecer hedges — it seems, it looks like, you have an impression.
ser
States a fact about identity or essence — you're committing to this being true.
Es interesante.
It's interesting.
A direct verdict. You're declaring it interesting.
Es una buena persona.
She's a good person.
Character as fact. Parecer would soften this to an impression.
Somos amigos.
We're friends.
Identity, category — what you are to each other.
parecer
To seem, to appear, to look like — your impression, not a definite fact.
Parece cansado.
He looks/seems tired.
You're reading the signs, not making a diagnosis. Much softer than 'está cansado.'
¿Qué te parece?
What do you think? / How does it seem to you?
Asking for an opinion or reaction. One of the most useful phrases in Spanish.
Parece que va a llover.
It looks like it's going to rain.
Impression from evidence — dark clouds, humidity. Not a forecast fact.
Tricky bit
'Me parece bien' = I think that's fine / it seems good to me. This is not the same as 'es bueno' (it is good). Parecer adds a layer of subjectivity. Very useful for being politely non-committal in Spanish.