Refers to a business that arranges travel, sells tickets, and plans trips. Can mean the company itself or a person working there, though 旅行代理店 is more precise for the office.
I booked a package tour at a travel agency.
彼は旅行会社に勤めている。
He works for a travel agency.
I booked a Hawaii tour through a travel agency.
He signed up for an overseas trip plan with a travel agency.
I arranged the hotel through a travel agency.
My older sister works at a travel agency.