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In this work high repetition rate femtosecond laser pulse driven x-ray plasma sources, created in an air environment using metal targets, are thoroughly examined and characterized. Due to contents of this work, it can be split into three distinct parts. In the first part, characteristics defining laser radiation and plasma relations (plasma frequency, pulse penetration depth, plasma density scale-length etc.) are described, along with the various mechanisms of pulse-to-plasma energy transfer. The second part of this work is dedicated to the principles of x-ray photon creation in a plasma source: the concept of hot electrons is introduced, as well is the generation of line and continuous spectra matching radiation. The influence of air environment to photon generation process is also discussed. The most extensive third part is used to present the creation of x-ray plasma sources of metal targets (via application of Pharos PH1-20 femtosecond pulse laser and two, slow and fast target renewal systems) and to describe and discuss the results of performed experiments (specifically, measurements of generated photon flux dependence on laser pulse energy, repetition rate, polarization, wavelength and angle of incidence in respect to target), as well as to compare said results to those found in other authors’ works. The data presented in this study demonstrates that high pulse energy lasers typically used for plasma X-ray sources can be efficiently replaced by high average power laser radiation, as a way to increase both the brightness and the repetition rate of the generated X-ray pulses.