Corfu 05 lectures - part I: Strings on curved backgrounds

In these introductory lectures we summarize some basic facts and techniques about perturbative string theory (sections 1 to 6). These are further developed (sections 7 and 8) for describing string propagation in the presence of gravitational or gauge fields. We also remind some solutions of the string equations of motion, which correspond to remarkable (NS or D) brane configurations. A part II by Emilian Dudas will be devoted to orientifold constructions and applications to string model building.