A pro-drop language is a language in which certain pronouns can be dropped from a sentence when they can be inferred. Spanish is a pro-drop language in the sense that you can say "Compré un libro ayer." instead of "Yo compré un libro ayer." In this sentence, "yo", meaning "I", is optional since the é ending on compré indicates clearly who the subject of the action is. Japanese is another pro-drop language. In Japanese, however, you can even drop subjects and objects that are whole noun phrases when they are clear from context. You can also drop direct object pronouns. As far as I know, in Spanish, you can only drop subject pronouns. --please correct me on that if I am wrong.-- So in Japanese, if you are talking to someone about a cake and want to ask who baked it, you can ask 誰が焼いた？ (Dare ga yaita?) instead of 誰がそれを焼いた？(Dare ga sore wo yaita?) Note that それを (sore wo) meaning "it" has been excluded from the question since it is obvious that you are talking about the caking having been baked. I am learning now that even Chinese allows for some pro-drop features. Languages like English, German, and French are considered non-pro-drop languages since nothing can be dropped.

Recently, I have been thinking over this in terms of American English though. At least, it seems to me in certain inform contexts, American English can exhibit some pro-drop features. For instance, a common informal phrase in American English is "will do". You can hear people say this when someone asks them to do something. Perhaps some people would say it is a lazy way of saying "I will do that/it." or "I will do that/it for you.", but even still, the subject pronoun "I", the direct object pronouns "that" and "it", and even in the latter sentence, the "for you" are being dropped since they are pretty clear from context. By the way, the definition of will do is here http://dictionary.reference.com/browse/will+do

Another example of pro-drop that I have encountered in American English happens in chat. Say you are talking to someone online and they suddenly disappear for 15 minutes. When they come back, you might find something like "Sorry. Had to get a drink of water." or "Sorry. Had to answer the phone." in your IM window. Just like with "will do" the subject pronoun "I" has been cast off the sentences because it is clear who had to do something and you are giving a reason for your disappearance. People also do this on the phone. For instance, if someone asks you to hold for a minute and they switch over to the other line, when they come back, they may say "Sorry. Had to answer my mother's call." instead.

This isn't only with actions though. I have even seen and heard it with adjectives too. For instance, say someone asks you in person or over chat how you are doing. You then respond with "I'm doing well." The other person might say or write "Glad to hear it." instead of "I'm glad to hear it." In this case, more than the subject pronoun is being cast off though. Look at how the word "am" is tossed out too. Hehehe. :)