Arduino Compatible can mean many things, but most of the time it means that some company has made something they sell to a customer that supports development with the Arduino IDE.
The thing is that most Arduino Compatible units use the Atmel processors, but some use others too. Third party developers then make "board"-projects that you can add into the Arduino IDE.
Be aware though that you in some cases can run into issues if you dont do your homework, here are some of the issues you can run into:
- Some "compatible" boards use different architectures, so even if you can do many things with the Arduino IDE, maybe some sensor libraries are not supported. (Might have to be hacked or updated)
- Some "compatible" boards have different USB-circuits (if they have them at all)
- Most of the boards you buy off aliexpress, banggood etc. do not come with instructions at all and some times they differ from the "original" design (might be better, might be "the same" or might be worse.).
I'm not trying to scare you off, but just my experiences. You can find a lot of fun things on those sites like clones of lilitiny with the AtTiny 85. One example is in my blog heer: http://mjelde.blogspot.no/2015/12/lilytiny-lilypad-attiny85-wearable-iot.html
The main issue with the clones I've had is that some times they differ in some ways that the first times will make you scratch your head. Like: ESP8266 is Arduino Compatible with the boards manager added ( https://github.com/esp8266/Arduino ), BUT: The input pins are not "correct" marked. So you should then add constants to "remap" them in your code.
Also the ESP8266 does only support 1V analog read, while many Arduinos support 5V. But you can "fix" this with an 16bit ADC (analog to digital converter). The ESP8266 only has 3.3V also, so you cant run sensors that "must" have 5V, without then giving them a different power circuit :)
The AtTiny 85 LilyTiny clone i bought in a pair of 2 from AliExpress works fine and works different than the rest of the Arduinos. Uploading to those for instance means you leave them disconnected and then you hit upload in the IDE. Then it says in the status window to connect it, which you then do.
Then it uploads. It has very few inputs, but it's very cute id say. Primarily it's made for wearable and of course it has a lot of short comings compared to a large arduino board. But you can of course use it to control RGB leds, etc.
Remember to always check specifications (3V vs 5V, how much analog read resolution can it handle, etc). There is the sensor vs board (some sensors work on both 3.3V and 5V, some work only on 5V, some only on 3.3V).
For going to 5V to 3.3V you usually use a voltage divider. Either you can buy pre made ones or you can make it based off three resistors and some math :-)