If you build a custom rom by yourself, after you've built a custom rom you can launch it with the command emulator. But to do this you need at first to build your rom for emulator (usually, full-generic lunch option). If not you can just copy necessary images and also run them using emulator command. For instance, I usually copy .sh script in the folder with Android sources and run this script:

out/host/linux-x86/bin/emulator -sysdir out/target/product/generic/ -system out/target/product/generic/system.img -ramdisk out/target/product/generic/ramdisk.img -data out/target/product/generic/userdata.img -kernel prebuilt/android-arm/kernel/kernel-qemu -sdcard sdcard.img -skindir sdk/emulator/skins -skin WVGA800 -scale 0.7 -memory 512 -wipe-data -partition-size 200