While travelling, a person joins travellers to help, explain and introduce the place we travel. What is the English word for that person?
There are a number of related roles
A guide or tour guide or more formally tourist guide (as mentioned) takes groups of tourists around, talking knowledgeably about history, architecture, etc. They may escort a tour group on their travels through multiple destinations, or just provide short tours of a specific city, area, or building. (Career information)
A holiday rep (short for representative) is a related job which involves providing assistance and possibly entertainment to holiday-makers and maybe arranging coach trips and transfers, but doesn't require any deep historical or art-historical knowledge. It's often associated with package holidays, beach holidays, etc. (Career info)
Another related job is a concierge, who usually is based in one place such as a hotel or a specialist concierge service, and who tells people the best places to go for whatever they want, but doesn't accompany them. (Wikipedia)
A tourist information officer does something similar to a concierge but works out of a (often publicly-funded) tourist information office and serves all visitors not just those who've paid.
This is the job of a tour guide (American English) or (less commonly used in British English) tourist guide. Just guide may work too.
a person who takes people on trips through an area and explains the interesting details about it
It can be used both for people who guide an entire multi-day trip for a fixed group of people, as well as guiding a 'random' group of visitors to a single location for a few hours.
To the terms already offered in the other posted answers, one should add courier. The term is, however, used in this sense only in British English; the speakers of American English are likely to find this use of it confusing. A courier works for a company that sells package holidays and represents it 'on the ground'; the term would thus not be used for the guides that work for local businesses.