Toki Pona is a conlang created by Canadian linguist Sonja Lang in 2001. With only 120 official words, the language is inspired by Taoism, and designed to get rid of unnecessary complex thoughts. In a way, it can be seen as a tool for mental health and mental hygiene.
There is no gender, no plural, no tense. Words can generally be used as nouns, adjectives or verbs. Unlike logical languages like Lojban, Toki Pona relies heavily on context and is highly ambiguous. Most words have a wide range of meaning, yet the minimalistic grammar makes its easy to learn and use.
Toki Pona uses only fourteen letters and is usually written with latin characters, but it also allows different scripts, such as sitelen pona, where each word is a simple glyph, or sitelen sitelen, a non-linear calligraphic system relying on complex Mayan-like blocks.
Many online resources are available to learn Toki Pona. Urandom's course is a great start. However, while reading the official book is not mandatory, it is highly recommended to get a true understanding of what the language is about.
Several of my projects involve Toki Pona, including musi lipu, pimeja, unpa pona, music, drawings, and more recently, ma kasi insa. For anyone interested in learning the language, I maintain a learning resources thread.
There's an endless war between "pu-rists", the people who use only the 120 words from the official book, also known as "Pu", and others, who enjoy inventing new words every now and them.
In my opinion, new words kind of defeat the purpose of the language. While a few of them might actually be justified for important concepts, such as "tonsi", which refers to non-binary persons, I find most additions superfluous.
The interest of Toki Pona is to embrace its minimalism: you have to think with a limited vocabulary, and twist your mind around the words it lacks to reach simpler or smarter ways of expressing your thoughts. Poetry, unknown feelings and introspection may arise.
While inventing new words can be fun, I think it also misses the point. Yes, a living language does evolve, but if Toki Pona is meant to simplify your way of thinking, you can't complexify it to suit your mindset.
However, the language isn't perfect, and it doesn't aim to be. It embraces its own quirks, and favors being playful over being coldly efficient. In this regard, I used to think that some official words were rather useless, such as "mu", or even "pu", or redundant, like "suno" and "mun", and could be replaced with words bearing broader meanings.
But since the release of "Ku", the Toki Pona Dictionary, I see things a bit differently. Words created by the community have been sorted and included depending on their popularity, and of course, it makes sense to recognize them. Using them or not is simply a matter of personal preference, in the same way the dictionary suggest translations as a range of possibilities without enforcing any of them.
Ku includes words that break the Toki Pona grammar, such as "yupekosi" (the letter Y doesn't exist in Toki Pona) or "Pingo" (which is capitalized but refers to a regular noun.) On one hand, my love for symmetry and finely tuned systems hates this. On the other hand, it's just plain fun, and clearly one of the reasons of Toki Pona's popularity.
On July 19th, 2021, Sonja Lang has released a new book, the Toki Pona Dictionary, also know as "ku". It's a 400-page two-way English/Toki Pona dictionary that adds 17 popular words definitions, which were for the most part semi-official, known as "nimi ku suli". It also broadens the language with another set of less known words, the "nimi ku pi suli ala" for a total of 181 words (including "ku"). Lots of these are jokes, or very rarely used.
I'd say adding new sets of words without making them officially "pu" was the smart thing to do, as it expands the vocabulary without diluting the core of the language. I still think some are really not needed, but at the time, they also gave me a better insight on the spirit of Toki Pona: no matter how more elegant or efficient it could be, it's meant to be fun!