What a haiku is
Haiku is a Japanese form, so anything we do in English is an approximation.
It consists in Japanese of metrical units consisting of 5, 7, and 5 syllabic units, which is why people sometime assume that it should be three lines written using 5-7-5 in English syllables, although that is not, strictly speaking, a correct assumption. Nevertheless, if you choose to define the syllable count as 5-7-5, that's fine, though it could be something like 3-6-4 or 4-7-3 or some other mix of short-longer-short that makes sense.
Haiku isn't just about length and syllables, but has a few other requirements if we are technically talking about haiku:
1. It should be based in nature and/or natural observation, and not be about people (per se).
2. It should contain a kigo, which is a seasonal indicator. This can be something like "April" or "spring", or something like "snow" or "ice" (which assumes that it's a reference to winter), or even something like "baseball", which is seasonal and association with spring/summer.
3. It should contain some sort of insight or realization (which is why I said it's not about people per se up front, since sometimes these turns/insights/realizations can be about an emotional impact).
4. It should be based in the poet's experience of the natural world around her (or him), and offer some sense of freshness on things.
5. Often, haiku should involve some form of "cutting" - usually at the end of the first or second line, often indicated by a dash or colon. The haiku, short as it is, consists of two independent parts that are nevertheless linked and enrich one another.
Here is one of the most famous in translation, by a famous Japanese poet called Basho (not his given name):
An old pond!
A frog jumps in-
The sound of water.
Here's another in translation, from Hashin:
No sky
no earth - but still
snowflakes fall
Tomorrow, a closer look at the kigo.