The objective of the KArlsruhe TRItium Neutrino experiment (KATRIN) is a direct measurement of the absolute mass of the electron (anti)neutrino by means of a precise study of the endpoint region of the tritium beta spectrum. The expected sensitivity of KATRIN to the neutrino mass is 0.2 eV (90% CL). The experimental set-up consists of four main parts: a source of electrons from tritium beta decay, a pre-spectrometer, a unique electron spectrometer with very high energy resolution and a multi pixel detector for low energy beta particles. A "Windowless Gaseous Tritium Source" (WGTS) with differential pumping of tritium is foreseen as the main source in KATRIN. This kind of source represents a gas dynamic system with the source tube 90 mm in diameter and 10 m in length placed in a strong magnetic field and differential pumping stages at both ends of this tube. Tritium gas will be injected in the centre of the source tube producing a gas flow directed to the ends. After pumping down by the differential pumps, compressing up to several hundreds millibars by the transfer pump and purification on the palladium membrane filter, tritium will be injected back to the source tube. The estimated flow rate of the circulated tritium is 1.8 standard cubic centimeters per second (sccs), which corresponds to 2.4×10-4 g/s (40 g/day). The stability of gas density and isotope composition in the source tube should be provided on the level of 0.1%. This paper will describe the design concept of the WGTS and will discuss the tritium processing techniques associated with the KATRIN experiment.