Femtosecond lasers that appeared in the 1980s have since undergone numerous developments, while the scope of their applications has broadened. The first applications mainly exploited the duration of femtosecond pulses to improve the temporal resolution of spectroscopic experiments seeking to observe the very first steps of physical or biological processes. Since then, a wide range of applications have emerged, taking advantage of the remarkable properties of femtosecond pulses. The concentration of the energy, in space and time, has led to many scientific and industrial developments from material processing to new imaging techniques to name a few. The very particular characteristics of the spectrum of these pulses, equivalent to a frequency comb, have also revolutionized metrology. In this docu- ment, we present the properties of femtosecond pulses and provide some tools to describe them. We briefly present the central concept of spectral phase and discuss the key elements of a femtosecond laser cavity, namely modelocking and group velocity management. Some basic oscillator architectures are presented, keeping in mind that the world of femtosecond lasers is today extremely vast.