Sounds like you're new to structural engineering. Here's a good link explaining a lot of the basic concepts involved in structural engineering:
To answer your question,
Service Loads represent the
actual imparted loads on a structure.
Ultimate Loads take the service loads, and
amplify (increased typically, but not always) them to represent the loads we
design for.
For example, if I have a concrete floor that is 4" thick, it weighs 50 pounds per square foot (psf). This is called the "Dead Load." Let's also assume the use of this floor is a residential space, which requires a 40 psf "Live Load."
So... Our
Service Loads are
50 psf Dead Load, and
40 psf Live Load.
In Load and Resistance Factor Design (LRFD), we
amplify these loads to find our
Ultimate Loads based on a number of "Load Combinations" outlined in the local building code (which generally reference ASCE 7-10 in the US).
The most common "Load Combination" is: 1.2 * Dead Load + 1.6 * Live Load
So, our
Ultimate Load would be 1.2(50) + 1.6(40) = 124psf
Essentially, in LRFD, the
Ultimate Load is the
Service Load with a "Factor of Safety" included.*
*not an exact definition but I think it'll do for what you're asking.