Citra is the most promising 3DS emulator currently in development; the open source emulator constantly receives nightly build releases which constantly improve the already powerful software.
This is currently the best 3DS emulator available but there are several other real ones which can be downloaded on the PC; all other 3DS emulators available right now are fake.
Fake emulators can be very dangerous to computers and devices, they can contain viruses or other malware which can severely damage the device or infect it with a key-logger to collect passwords from users.
While not all fake emulators do this, taking a risk when installing an emulator is not recommended.
Installing real emulators should only be done so through the official channels, downloading files from other websites may give you the working 3DS emulator but it could also contain nasty malware such as a key-logger or a Trojan.
When installing a 3DS emulator always take care and be sure to research the emulator first before putting your device at risk of infection.