Does imessage work only iphone to iphone?

I'm been texting my friend, and I have an iphone, she I think has the samsung galaxy s3 or 4. Every time I send her a text, it goes through as imessage by default. The problem is she hasn't responded to any of my texts and I'm starting to wondering that she didn't get it at all. The only time she HAS responded was when she texted me and when I texted back, imessage didn't work and went through as a green text. So Does that mean I have to disable imessage and use SMS text instead?

Well I'm assuming that's the case… If it's an Apple product. Disable iMessage, then try again.

Look in contacts under her name and make sure it says mobile instead of iPhone. Because if it don't it will be a problem. I had the same thing with my friend phone.

Yeah you have to send her SMS. IMessage only works between iPhone/iPod/iPad/Mac. Usually, it automatically uses SMS with non-iPhone users and iMessage with iPhone users, but yeah just disable iMessage for now. You can also send an iMessage, hold down the message you sent, and tap "send as Text Message". After you send a few SMS messages back and forth, it'll probably just start automatically sending everything as SMS between you two. If not, just disable iMessage or try going to settings, messages, turn on "Send as SMS".

Yes possibly

Use SMS and not Imessage as they won't be delivered to a device that isn't an apple.