Gosh, that’s a hard one to sum up.
The different Unix flavors started from a common base and diverged according to company goals. The basic commands are the same, but the options may differ (like the ps command. In Solaris, ps -ef is the common way to display processes, but other flavors use ps -aux. The -aux options are there in Solaris, but you have to use the executable in the ucb path). Someone familiar with one flavor can get around in the other flavors, but there are small, annoying differences.
Other differences are major. AIX handles user accounts differently than Solaris. Solaris uses flat /etc/passwd and /etc/shadow files. AIX uses some form of a database that I, admittedly, don’t understand very well.
The flavors from the major vendors are tailored to their hardware. Solaris for Sun hardware, AIX for IBM, HP-UX for HP and IRIX for SGI (does anyone still use IRIX?).
Linux is not developed for a single vendor’s hardware, but there are still major differences between the distributions. There is a linux standards base, but distribution owners aren’t required to follow it. Most of them stay fairly compliant, but Red Hat, for example, places the Apache files in a non-standard location because they prefer to.
Startup files differ from distribution to distribution. They are going to be somewhere in /etc/rc.d, but after that it’s anyone’s guess until you actually trace it through. Grrr.
Memory management, file systems, optimizations, scalability, built-in features; these all differ from vendor to vendor. The Linux distributions are closer since they are built on a comman kernel (unlike commercial flavors), but even then, optimizations may vary.
The question is too hard to answer in a post. The difference is like automobiles. For the most part, they all have four wheels. You can count on them having a steering wheel. In the US, the gas pedal is on the right, brake in the middle and, if necessary, the clutch is on the left. But jumping car to car you can never guarantee where the darn windshield wiper controls are. Unix is like that.