I am totally new to AR and I searched on the internet about marker based and markerless AR but I am confused with marker based and markerless AR.. Lets assume an AR app triggers AR action when it scans specific images..So is this marker based AR or markerless AR.. Isn't the image a marker? Also to position the AR content does marker based AR use devices' accelerometer and compass as in markerless AR?
In a marker-based AR application the images (or the corresponding image descriptors) to be recognized are provided beforehand. In this case you know exactly what the application will search for while acquiring camera data (camera frames). Most of the nowadays AR apps dealing with image recognition are marker-based. Why? Because it's much more simple to detect things that are hard-coded in your app.
On the other hand, a marker-less AR application recognizes things that were not directly provided to the application beforehand. This scenario is much more difficult to implement because the recognition algorithm running in your AR application has to identify patterns, colors or some other features that may exist in camera frames. For example if your algorithm is able to identify dogs, it means that the AR application will be able to trigger AR actions whenever a dog is detected in a camera frame, without you having to provide images with all the dogs in the world (this is exaggerated of course - training a database for example) when developing the application.
Long story short: in a marker-based AR application where image recognition is involved, the marker can be an image, or the corresponding descriptors (features + key points). Usually an AR marker is a black&white (square) image,a QR code for example. These markers are easily recognized and tracked => not a lot of processing power on the end-user device is needed to perform the recognition (and optionally tracking).
There is no need of an accelerometer or a compass in a marker-based app. The recognition library may be able to compute the pose matrix (rotation & translation) of the detected image relative to the camera of your device. If you know that, you know how far the recognized image is and how it is rotated relative to your device's camera. And from now on, AR begins... :)