Why it exists. To say how someone or something looks or seems to you, without saying it is a fact. It's softer and more polite than a plain statement: 你看起来有点儿累nǐ kàn qilai yǒudiǎnr lèi sounds caring, 你很累nǐ hěn lèi sounds like a verdict.

Person / thing看上去 / 看起来很 + adjectiveor像noun

+你今天看起来很累。Nǐ jīntiān kàn qilai hěn lèi.You look tired today.

+这个菜看上去很好吃。Zhège cài kàn shangqu hěn hǎochī.This dish looks delicious.

+他看起来像大学生。Tā kàn qilai xiàng dàxuéshēng.He looks like a university student.

–你看起来不太高兴。Nǐ kàn qilai bú tài gāoxìng.You don't look very happy.

Key points

Same meaning for looks: 看上去kàn shangqu = 看起来kàn qilai. In everyday speech, 看起来kàn qilai is more common.
Add 很hěn, 有点儿yǒudiǎnr or 不太bú tài before the adjective: 看起来很年轻kàn qilai hěn niánqīng.
"Looks like" + a noun needs 像xiàng: 他看起来像老师tā kàn qilai xiàng lǎoshī.
Not in the book · for a guess about a situation, use 看起来kàn qilai: 看起来他今天不会来了kàn qilai tā jīntiān bú huì lái le "it looks like he isn't coming today". 看上去kàn shangqu is mainly for what you can see. Same family: 听起来tīng qilai sounds, 吃起来chī qilai tastes.

Common mistakes他看起来像很累。 → 他看起来很累。Tā kàn qilai hěn lèi. 像xiàng goes before a noun, not an adjective.
我看见她很年轻。 → 她看上去很年轻。Tā kàn shangqu hěn niánqīng. 看见kànjiàn means "see". To say "she looks", use 看上去kàn shangqu.
这个菜看起来是好吃。 → 这个菜看起来很好吃。Zhège cài kàn qilai hěn hǎochī. No 是shì before an adjective.