What is the difference between iMessage and SMS

SMS messages are short text messages transmitted over the cellular voice network.

iMessages are text or multimedia messages transmitted securely to other iMessage users using data, either cellular or WiFi. iMessage is proprietary to Apple devices.

MMS messages are multimedia messages transmitted using the cellular data network.

SMS and MMS messages are universal in that they are not limited to any particular manufacturer or operating system.

Yes.

"Blue bubble" messages get sent via the data nework and Apple's servers.

"Green bubble" messages get sent via your cellular provider.

To send a blue bubble message the recipient needs to have an AppleID to receive the message or reply to it.

Green bubble messages via your cellular carrier work with any mobile device capable of sending/receiving SMS or MMS messages.

If you have unlimited texting/messaging, you do not pay anything to send messages, but depending upon their cellular plan your recipient may to receive them.

If you send iMessages, there is no charge except for perhaps data charges to send or receive.

You do not have to decide; an iMessage will always be sent unless the user does not have an AppleID, in which case an SMS/MMS will be sent instead.

Not necessarily, but group SMS or MMS is a carrier feature that they can choose to offer nor not.

iMessage can send an SMS or MMS to a group but if your carrier does not offer the feature it would count as one message sent per group member.