FAT32 (standing for File Allocation Table 32-bit) is a file system standard which used to be used widely by hard drives everywhere until the release of ntfs. FAT32 is now more commonly used on flash drives, however, smaller media like MicroSD cards, still use the FAT file system to use all the space it can. FAT32 requires 1 Gigabyte of allocated free space to function properly. In other words, if you put a FAT32 file system on a 4gigabyte microSD card, you would only be able to use 3.
The main reason why FAT32 is used on flash drives rather than NTFS is because the original FAT file system cannot store files above 4 gigabytes. NTFS uses quite a bit more space than FAT because of it's structure and therefore not practical to use on media that requires a lot of free space (flash drives, external hard drives, etc)