No, it is NOT an F-15 variant. The F-18 is a completely different aircraft. The latest version, the F-18E/F, is only a few years old. The "C" model is one of the best dogfighters in the world, too, and it's fairly new as well. It would've been nice to replace the F-14, but sadly, it wasn't possible for a wide range of reasons. Stay tuned.
"Are these Russian planes not better than the f15-f18???"
Not with a U.S.-traine pilot at the stick, no.
The F-16 will continue to serve, as well, and is a savage fighter in its own right.
Recall, that in the past 35 years, NO American fighter has lost an engagement. Not one. A big part of our doctrine is WE WIN, always. We always have the best toys and the best-trained crews, too.
The F-22 will still have the F-16, -18, and -15 with it, and an outside chance of the F-35, although I still won't bet the rent on that one.