The desktop image allows you to try Lubuntu without changing your computer at all, and at your option to install it permanently later. This type of image is what most people will want to use. You will need at least 1024MiB of RAM to install from this image.
Install or Run Live, no need to install
64-bit PC (AMD64) desktop image
Choose this if you have a computer based on the AMD64 or EM64T architecture (e.g., Athlon64, Opteron, EM64T Xeon, Core 2). If you have a non-64-bit processor made by AMD, or if you need full support for 32-bit code, use the i386 images instead. Choose this if you are at all unsure.
32-bit PC (i386) desktop image
For almost all PCs. This includes most machines with Intel/AMD/etc type processors and almost all computers that run Microsoft Windows, as well as newer Apple Macintosh systems based on Intel processors.
System requirements for Lubuntu 18.04 LTS included a minimum of 1 GB of RAM, although 2 GB was recommended for better performance, plus a Pentium 4, Pentium M, or AMD K8 CPU or newer. The RAM requirements increased from Lubuntu 17.10. Point releases include 18.04. 1 on 26 July 2018 and 18.04.