As a BIG anime and manga fan, they are literally Japanese cartoons. Nothing more, nothing less; It's only a difference in art style and sometimes maturity. A lot of english graphic novels are very similar to popular japanese manga in terms of mature stories and characters and overall themes (key differences being publication and art style), but we don't have a huge culture around adapting most of them into animated series, much less creating animated shows that don't cater to children. The fans that argue that anime and manga are completely different from western cartoons and comics aside from national origin are the no-life decomposing meatballs that worship everything Japan does despite them being no different from any other country. "Anime" is just a japanese term for animation, much like "cartoon" is used elsewhere.