From what I understand (and hopefully someone with more expertise will arrive to deliver us from doubt) the Byzantine Empire's forces changed A LOT over its nearly 1,000 years of continuous existence. If you consider it merely the Eastern extension of the Roman Empire (which is what the Byzantines thought of it as) then it's history is even larger in scope.

That being said, the Byzantine Empire's military went through several distinct phases.

1. Late Antiquity During this broad era, the Eastern Empire's forces resembled very closely those of the Late Western Empire. Forces were composed primarily of either citizens of the empire or foederati (contracted groups, tribes and peoples who essentially owed military service to the Emperor in exchange for benefits like land, food, wealth and protection). The Easter Empire is known through the Notitia Dignitatum to have possessed significantly more cavalry elements than the West, though the Notitia's accuracy is a subject of debate regardless. A good source for understanding the Late Roman Army would be Hugh Elton's Warfare in Roman Europe, AD 350-425

2. Justinian Justinian, who reigned in the 6th century AD, is well known for organizing the greatest period of expansion of the Eastern Empire following the fall of the West. The army he used to do so had changed significantly in the last 1-200 years. Regiments were organized along smaller, more flexible lines, and the Greek language played an ever growing part in the Imperial Government. In some ways however, it resembled the Late Antiquity forces. Troops were either Palace guards, responsible for the emperor, professional field army troops used for campaigning, semi-professional border guards/garrison troops, auxiliary foederati as before and allied tribes/peoples. One significant change was the increasing use of Bucellari, troops who were essentially private body guards of whatever commanding general was in the field. It should be noted that these had been used before, but during Justinian's reign his generals like Narses and especially Belisarius took it to another level.

A good general source here would be Michael Maas' The Cambridge Companion Guide to the Age of Justinian

3. Thematic Army The next great change occurred with the advent of the Theme system. The Arab invasions of much of the remaining Empire forced them on the defensive - the remnants of the various field armies of the previous era were settled in various regions of the surviving Empire - these areas were called Themes and were essentially military districts. The settled field armies were still liable to be called up for duty, and performed satisfactorily for several centuries - the theme system remained in use for a long time. These semi-permanent armies were well equipped to deal with the ubiquitous raiding of the Muslim forces to their south and east, and eventually worked to actively expand the Empire.

A good source for this period (and all of the Byzantine empire in general) would be Warren Treadgold's Byzantium and it's Army, 284-1081

4. Komnenian After the fall of much of Anatolia (modern day Turkey) and it's constituent Themes, the Byzantine Empire was forced to change rapidly to deal with the resultant crisis. The Komnenian Dynasty is well known for its success in leading the Empire through a period of crisis, and then resurgence in both Europe and Asia. Combined with mercenary forces, (not always) friendly crusades and delicate diplomacy, the Komnenians were able to reclaim much of the lost territory in Anatolia. But the success was relatively brief - the loss of the central themes inherently weakened the Empire's military capability, and the increased reliance on Mercenaries did not bode well for the future. With the sack of Constantinople during the 4th Crusade, the subsequent splintering of the Empire into rump states and continued conflict with encroaching g Muslim and Catholic forces, the Komnenian restoration came to a definitive end.

5. Final stages While the Empire limped along after the sack for another 250 years, it was subject to continuous weakening and dismemberment. It remained relevant as the western terminus of the Silk Road, and its wealth meant it had some pull. But the subsequent rise of the Italian City States and the Ottoman Empire put the nail in the coffin. At the final Siege of Constantinople (the last gasp of the Byzantine Empire) many of the remaining soldiers were of foreign descent, or were palace guards. They gave the Ottomans a run for their money, but can hardly be compared to the solid, well trained forces of the Komnenians, the Themes or the even older professional forces of Justinian and his predecessors. A good book (somewhat pop history but I enjoyed it) would be 1453 The Holy War for Constantinople by Roger Crowley.

I hope that gives you some sense of the massive scope of Byzantine history, and that's only the military side. Needless to say over such a long period of time the military changed a huge amount as it learnt how to deal with different challenges of enemies, manpower, economics and political necessity.