The simple answer to this question would be "Gundam are giant robots that are piloted in some sort of fashion." but that answer doesn't really tell you much, especially if you aren't even familiar with the American counter part that is Transformers. Another way to answer this question would be "a mechanical, walking vehicle that is similar to the shape of a human" (even though there are some Gundam that have more of an animalistic shape). Gundam them selves are usually referred to as "mobile suits" and are inspired by power armors from the American novel Starship Troopers [thank you wiki].
You might be familiar with a little show that used to play on Cartoon Network called Megas XLR. While not the best example of an "american gundam like series" (in fact, this follows more of the format that Macross does considering the aliens versus humans and all) it is one that from my experience a lot of the 90s cartoon watching generation have seen. While there are also no model kits that I know of from this series, and while the series itself doesn't really follow the same format as Gundam does (heck, Megas XLR focuses more on fighting an alien race rather than fighting with humans who happen to just not live on Earth) it's easier to tell people Gundam is similar to Megas XLR because it is a bit more Gundam like than Transformers because there IS a human pilot rather than an intelligent life form that happens to take on the form of a giant robot.
I wouldn't mind fighting off aliens in a giant robot.
Want to know something interesting? The Gundam anime actually could be considered a flop- well, pre-model kit introduction. In Japan the anime (I'm of course referring to the first Gundam series, not any of the recent ones) was actually cancelled. Originally intended to be 52 episodes, the show ended at 43. It wasn't until Bandai started released kits based on the mecha designs that the show actually became popular.
Cartoon Network has played a few Gundam series over time most notably Gundam Wing and Gundam SEED. Oh, and lets not forget about that lovely little gem us Gundam fans love to hate- SD Gundam. You may have also seen Gundam 00 on Sci-fi (back before it got changed to SyFy.... sigh). Overall, Gundam has made it's way to American audiences.
Now, there is so much more I could go into Gundam wise even just to explain the different aspects such as manga and anime that compile what is modern day Gundam, but I'll spread those out into different entries. Thank you for reading the first part of what I'm calling GIS- Gundam Info Series (well tech the second but I'll be redoing the supplies part).