In the present thesis the relaxor properties of the tungsten-bronze ferroelectric material strontium-barium niobate Sr_1-wBa_wNb₂O₆ (SBN) are investigated. SBN is a very good experimental realization of the three-dimensional random-field Ising model (3d RFIM). The quenched random fields (RF) originate from charge disorder and can be enhanced by doping with Ce³⁺. They are responsible for the formation of polar nanoclusters in the paraelectric phase. In order to study the critical behavior of SBN, linear birefringence (LB) and second harmonic generation (SHG) have been measured as a function of temperature. Within an Ornstein-Zernike analysis of the LB data the autocorrelation function, <P²>, was determined, whereas the dipolar correlation length was obtained from the SHG data. They suggest that, due to its intrinsic disorder, pure SBN does not belong to the 3d Ising universality class. Doping with Ce³⁺ions, which seem to generate RFs, enhances the relaxor properties. The critical exponents v and y of SBN:Ce shift against those of the 3d RFIM. The domain morphology of SBN:Ce has been investigated by piezoelectric force microscopy (PFM). Fractal-like shaped zero-field cooled nanodomains are observed. Their size distribution can be described by a power law with exponential cutoff in accordance with prediction for the RFIM. It was measured for the first time in a RF system. The temperature and field induced evolution of natural and written domains has been studied with PFM, LB and SHG measurements. It reveals a very slow relaxation from a macrodomain into a depolarized multidomain state (and vice versa) even above T_C. This hints at strong pinning forces due to quenched RFs. They are also responsible for the observed aging in poled SBN and the field induced cluster percolation above T_C measured with SH-hysteresis. The domains can be considered as a "thick" phase grating for SH-diffraction. The Bragg-regime diffraction efficiency reflects the temperature and field induced change of the averaged domain sizes in the system.