Most GameBoy cartridges on the market today are Read-Only Memory (ROM) cartridges or they are a combination of ROM and Random-Access Memory (RAM). The RAM section is usually quite small compared to the ROM section. It is smaller because it only has to hold a minimal amount of information. Just enough to allow you to turn your GameBoy off and still retain information about where you were in the game. This is the first major cartridge group.
The second and final major cartridge group is the one where the majority of memory is NOT ROM memory. The cartridges in this group can be downloaded with a commercial game or with your own custom made GameBoy software. This group contains many different forms of technology that can fulfill this goal. The Primary memory in this group, 'usually' the largest memory but not necessarily always, may consist of Dynamic RAM (DRAM), Static RAM (SRAM), Electrically Programmable ROM (EPROM), Electrically Erasable Programmable ROM (EEPROM), Flash EEPROM, or an EPROM emulator. Since DRAM needs a lot of support circuitry and EPROMs require an ultraviolet light to erase them, why don't we just concentrate on SRAMs, EEPROMs, and Flash EEPROMs, and EPROM emulators.
SRAMs, EEPROMs, Flash EEPROMs, and EPROM emulators; which should technology should a programmable GameBoy cartridge designer use? There are different trade-offs here. Among these are package availability (is it available in a small size?), power requirements (will it eat up unnecessarily the GameBoy batteries?), support circuitry (is it complete or does it need extra parts?), and ease of use (Do I have to unplug it and plug it into another device to program it?).
SRAMs, EEPROMS, and Flash EEPROMs are all different technologies but the all act the same as far as the user is concerned. Because they all act the same, they might all possibly be given the generic term of 'SRAM cartridge' or 'SRAM card', even though this may not be technically correct. Most currently available SRAM cards, either commercial or in prototype form, require the card to be plugged into a programmer to download to the card. Once this has completed, the card is unplugged and then it can be plugged into the GameBoy, just like any other GameBoy cartridge.
An EPROM emulator on the other hand is usually a generic term for something that can be reprogrammed without requiring a plugging and unplugging action to complete this task. This cartridge, or cartridge with a module connected, is usually connected to a host computer which can reprogram this cartridge at will.
Which is best for you, an SRAM card or an EPROM emulator, depends on what you plan to use it for mostly. If you mostly want to play video games and maybe basically 'dabble' at trying to program the GameBoy then buy or make an SRAM card. If you want to do SERIOUS GameBoy programming then buy or make an EPROM emulator. (Hint: If your making one, why not use an old or defective GameGenie as a starting point? It could contain all of your circuitry if you are very good at compact projects!)