In this paper, we discuss several concepts of the modern theory of discrete
integrable systems, including:
- Time discretization based on the notion of B"acklund transformation;
- Symplectic realizations of multi-Hamiltonian structures;
- Interrelations between discrete 1D systems and lattice 2D systems;
- Multi-dimensional consistency as integrability of discrete systems;
- Interrelations between integrable systems of quad-equations and integrable
systems of Laplace type;
- Pluri-Lagrangian structure as integrability of discrete variational
systems.
All these concepts are illustrated by the discrete time Toda lattices and
their relativistic analogs.