It is generally accepted by poetic scholars that Matsuo Bashō (1644-1694) is the creator of what we now call haiku. His most famous haiku is his "old frog" one, which is so simple yet complex, there are at least thirty-two different translations of it and everyone agrees, none are quite right.
A haiku originally was referred to as hokku. They were the first verse of a longer poetic form, which was a series of collaborative linked verse between poets. The structure of a hokku was 5-7-5 syllables (or the Japanese equivalent, which are phonetic sounds). The hokku portion was eventually separated completely from the other two lines. As the Poetry Foundation has described it:
Not popularized in Western literature until the early 1900s, the form originates from the Japanese hokku, or the opening section of a longer renga sequence. In this context, the hokku served to begin a longer poem by establishing a season, often with a pair of seasonal images. Unlike the rest of the renga sequence, which was composed collaboratively, the hokku was often created by a single poet working alone, and was subsequently used as an exercise for students. Over time, the hokku began to be appreciated for its own worth and became distinct as a poetic form, formally mastered by poets such as Basho and Yosa Buson. [Bold added.]
In 1890's poet Masaoka Shiki started referring to hokku as haiku and separated them. He was of the belief that the haiku itself was an independent poem, complete in itself. At the start of the 20th Century, western poets and authors such as Ezra Pound, Carl Sandburg, James Joyce, and William Carlos Williams began experimenting with the haiku. Then, in the late 1940's and 1950's, the Beat poetic movement of Allen Ginsberg, Gary Snyder and Jack Kerouac, embraced it. Kerouac even suggested a change in the haiku form given the use of it in non-Japanese languages:
"A 'Western Haiku' need not concern itself with seventeen syllables since Western Languages cannot adapt themselves to the fluid syllabic Japanese. I propose that the 'Western Haiku' simply say a lot in three short lines in any Western Language."
Over the decades since there has continued to be an interest in haiku, particularly amongst children. Haiku are usually written in the present tense, which is rare in poetry and prose. It gives an immediacy which other poetic forms don't have. Traditional haiku have a connection to nature or a season. They often have a kigo, which is a seasonal word or reference to a specific time of year.
Haiku also try to evoke an snapshot of an image in the mind of the reader or listener. At the same time, the image can be understood in a completely different way. This is done by using a "cutting word" called a kireji to separate the images or thoughts. Where in the haiku the kireji is placed, is one of the stylistic aspect of haiku studied by scholars.
Here are some examples of the common ones listed on the kireji wiki page:
ka (か): emphasis; when at end of a phrase, it indicates a question
kana (哉/かな): emphasis; usually can be found at a poem's end, indicates wonder
-ramu or -ran (〜らむ/〜らん): verbal suffix indicating probability
-ya (や): emphasises the preceding word or words. Cutting a poem into two parts, it implies an equation, while inviting the reader to explore their interrelationship
As one might expect, there is some tension between the haiku traditionalist and those who prefer the more modern style of haiku. But the one thing they can agree on, is that haiku and senryū are two different things.