One of the particular problems of answering questions about what feudalism is, is the fact it has become saturated in meanings derived from a number of different sources. It takes a significant amount of critical thinking to untangle the various strands and what you mean by feudalism.

For myself, feudalism is an analytical construct used by historians to explain the historical phenomena they found in the Middle Ages from c.850. For others feudalism is synonymous with pre-Modern society which emerged from the rubble of post-Roman Western Europe. Some conjure up hierarchical structures imposed top-down by the king or emperor (typically an Anglophone tradition), while others envisage the breakdown of society into squabbling warlords seizing royal authority (a more Francophone tradition). For others it might be the period when Europe was dominated by castles, overrun by mounted knights, peasants or serfs toiling in the dirt. For many it is the feudal society which is centrally built around possession of a fief or fee in return for military service and the construction of incredibly powerful bonds based on these tenurial contracts, known commonly as 'vassalage'.

These images may be constructed from popular culture, reading Marx, or studying history in school or at a university. It is certainly not impossible to keep two conflicting images in mind at the same time. It is impossible to answer the question of how feudalism came to exist (if it did at all) and how it developed without knowing what feudalism is being discussed.

A very useful text on the topic is Levi Roach's encyclopaedia entry on the topic. For comparison is a slightly older (and unpublished) 'dictionary of the history of ideas' entry by Frederic Cheyette.