In Cali (California), we would typically say that about someone who is "posing" or trying to be something they're not. I haven't heard anyone really use that in over a decade, so I'm not sure people still use it. The word "fronting/frontin" is more commonly used in my area.

'Any one who does not follow his/her own ideas of what he/she likes.' E.g. Some people call singer Avril Lavigne a poser because she originally played rock music and now she is popular as a 'pop' musician.

- In general - it's someone who pretends to be someone/something they aren't. (usually to impress others)

"Poser" is a person who poses. A synonym might be a "pretender". "To pose" (intransitive verb) is "to affect an attitude or character usually to deceive or impress" (Merriam-Webster).