The most significant change Microsoft made to Windows 95 was replacing Program Manager with a more Macintosh-like UI, but notably featuring a menu button with the word "Start" on it.
This is why the Windows logo menu button is still called the Start Button today.
Microsoft marketed the heck out of it in a TV ad campaign 25 years ago today: https://www.youtube.com/watch?v=CVd_NyQ7-Aw
Windows 95 was the first version of Windows that didn't need MS-DOS preinstalled separately.
Instead, it installed its own version of MS-DOS together with Windows. (The roles of MS-DOS as kernel and Windows as shell weren't swapped on home PCs until 2001's Windows XP.)
Microsoft did this to simplify installation by end users, of course, but also in no small part to kill off the MS-DOS compatible OS market, a niche filled most notably by Digital Research's DR-DOS.
One last thing on this thread: The Computer Chronicles episode about Windows 95 after boxes were available: https://www.youtube.com/watch?v=ZBbvmORcgSo
Anyway, happy silver anniversary, Microsoft Windows 95!