  Before the launch of the Fermi Gamma-ray Space Telescope satellite only two
classes of active galactic nuclei (AGN) were known to generate relativistic
jets and thus to emit up to the $\gamma$-ray energy range: blazars and radio
galaxies, both hosted in giant elliptical galaxies. The discovery by the Large
Area Telescope (LAT) on-board the Fermi satellite of variable $\gamma$-ray
emission from a few radio-loud narrow-line Seyfert 1 galaxies (NLSy1) revealed
the presence of an emerging third class of AGN with powerful relativistic jets.
Considering that NLSy1 are usually hosted in late-type galaxies with relatively
small black hole masses, this finding opened new challenging questions about
the nature of these objects, the disc/jet connection, the emission mechanisms
at high energies, and the formation of relativistic jets. In this review, I
will discuss the broad-band properties of the $\gamma$-ray-emitting NLSy1
included in the Fourth Fermi LAT source catalog, highlighting major findings
and open questions regarding jet physics, black hole mass estimation, host
galaxy and accretion process of these sources in the Fermi era.
