Internet Service Providers connect you to the Internet, they are the companies you pay your bill to for renting their modem and router that sits in your house. Much like a phone, cable or utility company, your ISP has the physical infrastructure - cables, networking and routing equipment. They connect to other ISP's that serve content and websites that you view in your browser.
You might have noticed that many ISP's are phone and cable companies. Dial-up (phone wires), cable, DSL, satellite, fiber optics and much more are the physical mediums which data is transferred over. Use this handy website to see your ISP .
Tracking & Personal Data
Your ISP can see what you websites you visit, but not necessarily your passwords or information you send. The big concern about an ISP tracking you is the meta-data they sell about you. Trump signed a bill that allows for ISP's to sell user data without user consent.
ISP now stands for "Invading Subscriber Privacy" and "Information Sold for Profit".
Check out more details at Public Knowledge, a non-profit fighting for consumers and open competition.
Depending on your ISP and their terms of service, they could save your data for up to a couple of years. Verizon was caught inserting tracking cookies secretly into your requests
The only way to prevent your ISP from tracking you is using a VPN (Virtual Private Network) or TOR.
Sell Your Information
ISP's store your search records and sell your meta-data to advertisers
ISP's may insert tracking cookies into your search requests