We introduce the concept of dissipative solitons, which are the result of a balance among dispersion, nonlinearity, gain, and loss. We focus our discussion on dissipative solitons in passively mode-locked fiber systems, which can be described by the cubic-quintic complex Ginzburg-Landau equation (CGLE). The conditions to have stable solutions of the CGLE are discussed using the perturbation theory. We use also the variational approach to find approximate pulse solutions for the CGLE. The existence of both stationary and pulsating soliton solutions is indicated by such approach and confirmed through some numerical examples. In particular, we describe the dynamics of plain pulsating, creeping, and erupting soliton solution. Moreover, we discuss the impact that some higher-order effects, namely the third-order dispersion, intrapulse Raman scattering, and self-steepening, can have on these pulses. We show that, for some range of the parameter values, these higher-order effects can transform the pulsating solitons into fixed-shape solitons.