Since its first observational evidence, gravitational lensing - the deflection of light by mass - has become an established astrophysical tool than can be used to study the physical properties and environments of the lens galaxies and to reveal the structure of the lensed quasars. This thesis combines theoretical and observational results of a sample of gravitationally lensed quasars to study the structure of the broad-line region (BLR), the size of the accretion disk and the mass of the central Super Massive Black Hole (SMBH). We start with a general introduction and a theoretical description of the basics of lensing (Chapter 1) followed by a short motivation statement (Chapter 2). In the third chapter a summary of the observations, data reduction and lens database is provided. This chapter also includes a description of the analysis tools and methods used in this thesis. We continue with the study of microlensing variations in the light curves of the wide-separation lensed quasar SDSS J1004+4112 and the crossshaped lensed quasar HE 0435-1223 to infer the size of their accretion disks (Section 4.1). The main conclusion is that the accretion disks are greater than expected according to the standard Shakura-Sunyaev disk model. In Section 4.2 we analyze the impact of intrinsic variability and microlensing in the objects of our spectroscopic database concluding the existence of two different regions in the BLR, one that is insensitive to microlensing (of size 50 light-days and kinematics not confined to a plane) and another (of size of a few light-days, comparable to the accretion disk) that shows up only when it is magnified by microlensing. The strong microlensing of some spectral features indicates that they could arise from the inner parts of the disk. This may be the case of the Fe III UV blend, a feature which we found systematically redshifted. Under the gravitational redshift hypothesis, this feature could be used to estimate, in a way independent of the geometry of the system, the mass of the central SMBH (Section 4.3). Finally (Chapter 5), we summarize the main conclusions of the thesis and emphasize the work in progress and future prospects.