Both C, and a pre-compiled basic (no-Picaxe bootstrapped needs, use raw chip)
Aparently this way you can take your picaxe-projects, and take them to the next level if you need faster execution, wants to code in C, need floating point etc..
If you decide to discard the picaxe bootloader and use your chip as a standard pic then you need to get hold of a pic programmer for programming the chip. One of the nice things about the picaxe is that it’s very easy to build a programming circuit. My advice is to keep your picaxe bootloader on the chip and buy new raw pic chips if you want those (or look at Atmel or ARM) since you can’t reprogram your pic with the picaxe bootloader.