A linguist is someone who studies language. Linguists study every aspect of language, including vocabulary, grammar, the sound of language, and how words evolve over time.
The study of language is called linguistics, and people who study linguistics are linguists. Being a linguist isn't easy; you usually have to go to graduate school and conduct research about a specific aspect of language. There are computational linguists, forensic linguists, comparative linguists, and many other specialties. If you love language, you'd probably enjoy a career as a linguist.