I don't understand the iPhone iMessage?

I know that when you send green text bubbles that means its a standard SMS and when it's blue then it's iMessage but why do people say that the standard SMS can cost you money am I being charged? And is it possible that I text someone else with an iPhone and it still appears green

Does anyone understand iMessage haha. Unless you're not on a contract or have a certain amount of texts you will be charged for SMS. If you've got a text bundle then you're fine, no worries! If the text bubble appears in blue then it has been sent via internet (so if you have data in your plan then no worries your not getting charged either! But if you don't have data, then you will be sorry to say). Sometimes you're sending a text to someone else who has an iPhone and it normally appears blue but it has appeared green, that is because they may not be in an internet area so it has had to be sent via SMS.

Green means it's a standard SMS, which pretty much every modern mobile phone can send and receive. These cost money, and are charged based on your mobile plan with your service provider (like AT&T, Vodafone, O2 and so on).

Blue indicates that your message recipient can receive iMessages on their iPod touch, iPhone, iPad or Mac. This is done over the internet, so it will tap into your Wi-Fi or 3G. Data sent through 3G will contribute towards your mobile data usage. (comparatively it's much cheaper to use iMessage than SMS). See source.

It is possible that it will appear green if:
You aren't connected to 3G or Wi-Fi network
The recipient is not connected to a 3G or Wi-Fi network
The recipient hasn't upgraded to iOS 6