I am quite confused how Fichte's ideas (and Schelling's ideas) and Hegel's ideas differ. There seem to be some common elements, but I am unsure how to pinpoint the difference between Hegel's and Fichte's.
Fichte, Schelling, and Hegel were divided in most significant respects by their views about realism and naturalism, the status of subject-object identity, and the nature of rationality and intellectual intuition. The differences between Fichte and Schelling are explored in depth in Hegel’s Differenz des Fichteschen und Schellingschen Systems der Philosophie (for a more contemporary review of the issues you might look into The Philosophical Rupture between Fichte and Schelling: Selected Texts and Correspondence, 2012, edited by Michael Vater and David Wood).
To state things crudely, the main difference is that Fichte upholds a more or less Kantian interpretation of the subjective status of transcendental idealism, whereas Schelling and Hegel defend an objective interpretation of absolute idealism. In particular, Schelling and Hegel criticize in different ways the Cartesian tradition of foundationalist epistemology, where we need to begin philosophy from the presuppositionless starting point of the knowing subject or res cogitans. For Schelling and Hegel, however, this involves an unacceptable form of solipsism or dualism that is created by falsely abstracting the knowing subject from nature itself.
Roughly put, if we start from the subject and work outwards toward nature we are forced to postulate an unbridgeable gap of intelligibility between subject and object. The problem being that the transcendental ego cannot reach outside itself to make contact with an externally standing reality. In order to escape this both Schelling and Hegel attempted to reconceive the knowing subject in terms of the natural history of consciousness; the ultimate end of the organic powers of nature. Their claim boils down to the idea that if the subject is internal rather than external to nature then all of nature is contained within itself; a notion similar to Spinoza's substance.
Apparently, this form of idealism is called objective and absolute since subjectivity emerges from a pre-existing objectivity which is dissolved in a relation of self-identity; meaning that self-consciousness coalesces out of nature and returns to nature in order to become something that is neither nature nor self-consciousness alone, but is rather the undifferentiated unity of the absolute.
A very simple and schematic explanation would be the following:
Fichte is a subjective idealist, meaning nature is constructed by (minimally: dependent on) the (personal, finite) subject.
Schelling is an objective idealist, meaning nature is thought of as being the (creative) subject.
Hegel is an absolute idealist, meaning both nature and the personal subject are created by the absolute idea (read: absolute subject), which thus underlies both nature and the personal (finite) subject.
According to this scheme, absolute idealism is the synthesis of subjective and objective idealism.
The main question for all three is "What is the generative principle of the world?" Fichte saw it in the personal I, Schelling in nature (as subject) and Hegel in the absolute idea as a principle similar to the personal ego but all-encompassing and thus being the ground of both nature and subjective spirit.