ESL is English as Second Language
TESOL or TESL or TEFL are teacher certificates for ESL courses
Teaching English to Speakers of Other Languages (teacher in USA)
Teaching English as a Second Language (teacher in USA).
Teach English as a Foreign Language (teacher travelling)
From the perspective I am a USA resident and teacher, though British, Canadian and Australian residents can be considered English first without getting into the whole Quebec, Canada preferring French.
English as a Second Language is more an adjective, not a certification.
All certificates are fairly equal, TEFL is a superset, good for home and away. While TESL and TESOL are mostly for home.
Eslfriend.com is one of many ESL web sites. Certifications may be available at Arizona Course or PeopleU or ESL.
English as a volunteer.
If your student is learning English for a job, where they need to pass a test to acquire the job, they would no doubt expect you to be TESL certified as they as the student will need to pass their own student test.
If someone is an adult learner not fully understanding of English, the certification may not be so important, they want life skill than a job skill.
Instead of diving into a classroom to be certified, try it and see if you like it. Two very accessible places is you can teach for free at Watertownlib Public Library. One trip, two hours a week required. You can teach two students one hour each Thursday night from 6:00 to 8:00 as a tutor would be typical volunteer hours per week. They also give you two hours free teacher training to get you going.
There are opportunity to tutor in Cambridge. Instead of tutoring specific students, you could be a Teacher Assistant. The classroom classes are during the weekdays.
Also, general learning center is on Rindge Avenue, a walk from Alewife.
Places to teach online could be preply, italki, verbling and cambly.