Stargate's are implemented via Addon Pack (CAP) with some modification provided by Person8880.
Stargates in lore are transportation mechanics between two gates, and in the server they play a very similar role.
Overview
Stargates come in various flavours (Milky Way, Pegasus, Universe, Infintiy), however each behave in the same fashion. When spawned a Stargate will have a default address (6 characters long non-repeating), a galaxy (separate from TSCM galaxies), and a name (there are a few extra fields like local and blocked).
To connect a Stargate you must 'dial' it, most typically done via interacting with the Stargate itself (though there are other methods like remote dialing, wire, or the DHD). Simply double click on the address you wish to dial, stand back and then walk through.
Some players may have an 'iris' in place, which will instantly kill you if you walk through.