Hi,
As Eric says the iPhone has a Messages app all of it's own.
It can do two things.
Send iMessages to Macs and iOS devices and also SMS to non Apple devices (Android Phone effectively).
However the iMessages side as it were requires that it has a good enough Carrier service to send iMessages or is on a WiFi network.
If it is Not it will resort to Sending SMS messages instead.
The app can whilst on a suitable service try to send an iMessage to a Number but will get info back from the iMessages server that it is not Registered and will send an SMS text instead.
Therefore it should automatically send SMS to your Android.
iMessages can be sent to Apple IDs or to iPhone Numbers both from Macs and from iOS devices.
You would normally refer to the Contact Card for that person and the select the Apple ID or iPhone number. Many would chose a Name from the Contacts App (Address Book ) and Send - somewhat hoping the right option had been selected or forgetting that not everyone with a Mac uses an iPhone and try to send anyway.
Therefore your Mac may get iMessages sent to your Apple ID.
There can be issues if you had an iPhone at one time and now the number is used on a Android or other type of phone.
This is because the iMessages server may think the device is still an iPhone if you did not disconnect it from the Service.
In this case the Mac > Messages > Preferences > Accounts > iMessages account may still list an "iPhone" Number, in the Receive At list and Start Conversation From drop down, which some people may still be sending iMessages to.
You may well have to "educate" them as to where you want Text Messages to go.

9:02 pm Monday; January 19, 2015
iMac 2.5Ghz i5 2011 (Mavericks 10.9)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad