Also I still dont understand why companies with their own software departments are asking Accenture to work for them. If Accenture employees are often substandard why not just hire more devs themselves?
Legacy systems: Any old software systems, usually enterprise IT, that are running your organization. Often 80's era in big companies, usually on a mainframe and most importantly, even though they are held together by hopes and prayers they usually do the job they are supposed to.
Dev shop : derogatory term for a company who just provides developers, usually with poor oversight and project management controls.
The problem with hiring more people is if I need 20 Devs for six months in a language I am never going to use again, not only do I need to hire 20 Devs, I need to hire the project managers, people managers, pay for their benefits or contracts, and then figure out how to fire or otherwise reallocate people with skills I don't need.
Re the terms you want explained, just use your favourite search engine. There are legitimate and established definitions for these things. Eg, your bank likely uses Cobol from the 1960s in its backend and they would dearly love to replace it because all of the original source is long gone. This is a legacy system.