Several parts combine into one character.

雲 has a small archaeological surprise built in. The earliest oracle bone form was just 云 — a curling line representing rolling clouds. Centuries later, scribes added the 雨 radical on top to clarify "the kind of cloud that brings rain". Then in the 20th century, Chinese simplification stripped the 雨 back off, returning the character to its three-thousand-year-old original. So today: 繁體/新字体 use 雲, 简体 uses 云. The same character circled back to its source.

Know both forms. In Japan you read 雲; in mainland China you read 云; in Taiwan and Hong Kong you read 雲 again.

Mandarin: yún, rising 2nd tone. 白云 (báiyún, white clouds), 乌云 (wūyún, dark clouds), 云海 (yúnhǎi, sea of clouds — a famous mountain-top sight). Beware homophones: 云 is also a classical Chinese verb meaning "to say" (as in 子曰诗云, "the Master said and the Songs say") — modern Chinese rarely uses this sense outside literary citation. Cloud-computing in tech is 云计算 (yúnjìsuàn) — the metaphor translates intact.

Japanese: on-reading ウン (un) in 雲海 (unkai, sea of clouds), 風雲 (fūun, "wind and cloud" — a metaphor for stormy times of upheaval). The kun-reading くも (kumo) is the everyday word — but watch out: 蜘蛛 (kumo, spider) is a homophone, distinguished only by context.

Memory aid: rain on top + a swirling line below = the sky-thing that produces rain. In simplified, the rain disappears and only the swirl remains.