You're a professional photographer. You're talking to someone who's not a photographer about your industry. You are discussing how difficult the field of photography is:
There's fierce competition among photographers for clients.
When people or groups work hard to win against each other, that's called "competition". "Competition" in business means the idea of companies working to get money and customers before other companies get them.
When companies that sell similar products are competing hard against each other, one way to describe it is to say that the "competition is fierce". "Fierce" means energetic, wild, and maybe a little bit violent. It's usually used to describe animals, but it can also describe the level of competition between two people or companies.
"Between" is used when you're talking about a relationship between two things:
I'm having a hard time choosing between getting a Mercedes or a BMW.
"Among" is used when there more than two things have a relationship:
There was some kind of disagreement among the board members.
"Clients" are similar to "customers". But "client" is more often used in business-to-business sales. "Customer" suggests someone buying things at a store or directly on the Internet.
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