Google DuckDuckGo is your friend :)
https://wiki.ubuntu.com/Lubuntu#Lubuntu_VS_Ubuntu (emphasis mine):
Lubuntu VS Ubuntu
Both Lubuntu and Ubuntu share Two Major Important Things:
- Same Core System
- Same Repositories
Lubuntu and Ubuntu belong to the same family and talking about each as
totally different two systems is not correct since they have some
things in common. Thus, we use the same Forum Area and share many Wiki
The differences between Lubuntu and Ubuntu are:
- Different DE - Lubuntu uses LXDE while Ubuntu uses Unity as the default DE.
- Different Default Applications
Other than that, they are the same. The DE is what makes Lubuntu a
lightweight OS, and of course the selected applications too because we
make sure to use the lightest applications which are not resource
Also, as a general rule, if you want a lightweight distro don't go for Ubuntu. Any flavor. Go for a real light weight distro like damnsmalllinux, or Arch.