There are some differences between Starcraft 64 and Starcraft (PC). Some of them are very easy to see, and others are harder to see. Some are major changes, others are minor changes. In the following paragraphs I will explain as best as I can about the differences, yet I might leave some differences out that are very minor and nobody would really care about.
The major differences between Starcraft 64 and Starcraft (PC) (besides the obvious) is the fact that Starcraft 64 has all 58 missions from Starcraft (PC) and Brood War(PC). This feature is really handy becuase now you can play more missions with only one cartridge. Of course, you must remember now that in order to play the Brood War missions on Starcraft 64 you must have a N64 Expansion Pak. You can find these at any retailer that sells video games and such.
The next difference is that in Starcraft 64 you don't have the Campaign Editor feature as you do in Starcraft (PC). This means that you cannot make custom maps. Although you cannot make custom maps, you do have the ability to customize the opponents and special requirements in Custom 2-Player Missions.
The maps and story lines in Starcraft 64 are the same as in Starcraft (PC). Although the stories and maps are the same, memory constaints required a few changes in perspective and number of units. This does not make Starcraft 64 easier though. In some cases this might mean that it is harder.
Another difference (that you might have guessed) is the controls. I am talking about how you can assign up to four groups of units to individual buttons, the capability of issuing destination commands to specific units (when this is ordered the units will go straight to the destination and will not stop for any enemies) and how you can quickly find all of one unit type. These strategies will become very useful when you play against others.
The next change in Starcraft 64 I found to be very useful. In Starcraft (PC) you can highlight up to 12 units at a time, but in Starcraft 64 you can highlight up to 18 units at a time. This is better on attacks or on defense becuase 18 Hydros are more frightful then 12 Hydros (power in numbers).
One thing about the Starcraft 64 units is they seem more aggressive. What I mean is that they will get into more fights than in Starcraft (PC). This is due to the fact that you can tell a unit(s) to go straight to a destination point (usually good for retreating), or to go to a destiantion point and if they see any enemies on the way they will kill those units, and then conitnue on the way to the destination point. This is better than Starcraft (PC) becuase you could only tell units to go to a certain destination.
One useful function (but I always seem to forget it when playing) is that you can assign certain sections of the map to hot keys. This is very useful if you have more than one base, or for any other reason. To set a hot key, use the mini-map to select the desired area of the screen. Then, hold down [Z] and press one of the four buttons on the D-Pad. Repeat the process to program all four buttons. You can return to any of these areas by simply pressing any button on the D-Pad.
Two minor changes between the Starcraft games is that in Starcraft 64 Drones, Probes, and SCVs will automatically start gathering minerals and/or harvesting Vespene Gas. Once they are built, they will split themselves up between minerals and (if a Extractor, Assimilator, or Refinery is built) Vespene Gas. In Starcraft (PC) the Drones, Probes, and SCVs just stood around your base until you give them a certain command. The other small difference is that in Starcraft 64, a Protoss Shuttle must come to a complete stop before unloading units. In Starcraft (PC) Shuttles may still be moving when they unload. This is one of those who cares difference, but I just thought I'd throw it in here anyways.
Overall, Starcraft 64 and Starcraft (PC) are alike in many ways, but they also have some small changes to them. After playing Starcraft 64 for awhile you will most likely get used to the changes and not even know they are there