Selamat Datang...Selamat Datang...

Living in a bilingual (or multilingual) community forces people to be able to speak in at least two different languages. I will take Javanese as a case in point. Javanese people can speak Javanese, their mother tongue, and Indonesian as their secondary or national language at a minimum. It is possible to find them speaking foreign languages too. As people have to speak different languages (or follow different speech levels; i.e. ngoko or krama in Javanese language) for different reasons, the so-called linguistic phenomena of code switching (you call it "alih kode" in Indonesian) and code mixing (campur kode) will inevitably occur.

When you alternate between two (or more) languages during your speech with another bilingual person, that means you have code-switched. Here is one of my favorite examples of code switching (cited from http://www.apfi-pppsi.com/alihkode.html, my translation):

A: Yanis, tu veux du "gado-gado"? (1) (Yanis, would you like gado-gado?)
B: Mais oui, je veux aussi du "es dawet". Quand on travaille dur, on a toujours faim. (2) (Yes, I’d like es dawet too. When we work hard, we are always hungry.)
A: Pak Mar, tolong pesen gado-gado kalih, es dawet kalih. (3) (Pak Mar, please bring us two gado-gado and es dawet.)
C: Inggih, inggih. (4) (Alright.)

You can see speaker A changes her language from French to Javanese (in utterances 1 and 3). She speaks French when talking with her friend B because her friend knows French too, and then switches to Javanese to talk to C as C does not speak French.

As for code mixing, it occurs when you incorporate small units (words or short phrases) from one language to another one. It is often unintentional and is often in word level. You probably say or hear someone saying something like "jangan suka nge-judge gitu dong. orang kan beda-beda" (note that "judge" is the English word inserted in the Indonesian utterance). You can see that in code mixing, you don't alternate the whole sentence, but you only use one word or two. This often happens unintetionally. Sometimes you have a bunch of lexicons that get jumbled in your brain, and you often use more than one languages.

Now you can see the difference between code switching and code mixing. When you change language intentionally and you do it because of specific purposes (e.g. the presence of third person that does not share the same language, or the change of topic or situation), in other word the switch is functional, that means you code-switch. When you insert a piece of word other than that of your language, and you have no specific purpose or intention when doing that, that means you code-mix.

I have done two researches on this subject. One is on code switching in a home-office domain, the other is on code switching in a mailing list forum. Pretty interesting :D.