It is a web based tool that can be used for test-case management,track and organizing purpose.
Pros: i have using this tool for my every day test execution before this i was using our own company proprietary tool and then we switched to TestRail as we switched to agile methodology. i have found this quite useful in agile environment. it is scalabe and can be customised accordingly to suite the customer/test requirement.
it is now integrated to our companies other tools like JIRA(bug tracker tool),github,automated test suites, bitbucket etc and many others and is working flawlessly. Reports generation and tracking the various result is quite simple and have real time insights for these with this tool. We can easily link our test cases, requirements, use cases and stores using this.