What kind of artificial or invented languages exist. And are either of them still in use?
I have heard of Esperanto, Ido, Volapük and Toki Pona. But I have a feeling that there probably are a lot more than those.
Do either of them serve a practical purpose?
Yes, there are many artificial or constructed languages that have been created over the years. Some of the most well-known ones, in addition to Esperanto, Ido, Volapük, and Toki Pona, include:
1. Klingon: a language created for the Star Trek TV series and movies
2. Elvish: a language created by J.R.R. Tolkien for his Middle-earth stories
3. Lojban: a language designed to have a logical structure
4. Interlingua: a language based on common roots of European languages
5. Novial: a language designed to be easy to learn for speakers of multiple languages
Some of these languages are still in use today, although their usage may be limited to small communities of enthusiasts. Esperanto, for example, has a community of speakers and is used for international communication, while Klingon is used primarily by Star Trek fans.
The practical purpose of these constructed languages varies. Some were designed to be universal and facilitate communication between people of different languages and cultures, while others were created as artistic or intellectual exercises. Some constructed languages were designed to be more efficient or logical than natural languages, while others were created to explore the limits of language as an art form. Overall, the practical purpose of these languages depends on the intentions of their creators and the communities that use them.
Tell me more about Novial. What defining features help it to accomplish its purpose?
Novial, an abbreviation of Nov International Auxiliari Lingue (New International Auxiliary Language), was created by Danish linguist Otto Jespersen in 1928. It was designed to be an easy-to-learn constructed language that could serve as a bridge between speakers of different languages, specifically those of the Indo-European family.

Some defining features of Novial that help it accomplish its purpose include:

1. Simplicity: Novial was designed to be simple and easy to learn, with a small set of grammar rules and regular conjugations. The language avoids irregularities and exceptions found in many natural languages.

2. International vocabulary: The vocabulary of Novial is drawn from a variety of languages, primarily English, French, German, and the Romance languages. This makes it easier for speakers of these languages to learn and recognize Novial words.

3. Regular word formation: Novial uses a regular system for word formation, which allows speakers to easily create new words by combining existing roots, prefixes, and suffixes. This feature makes the language more flexible and adaptable.

4. Clear phonetic spelling: Novial uses a phonetic spelling system, with each letter representing a single sound. This makes pronunciation straightforward for learners and reduces ambiguity.

5. Flexibility in word order: Novial allows for flexibility in word order, which can accommodate the preferences of speakers from different linguistic backgrounds. It does have a default subject-verb-object (SVO) word order, which is common in many Indo-European languages.

6. Gender neutrality: Novial eliminates gendered nouns and articles, making it a more inclusive language that avoids gender bias.

These features of Novial aim to make it an accessible and practical auxiliary language for international communication. Although the language never gained widespread use, it remains an interesting example of a constructed language designed for ease of learning and communication.