PS3HaX Homebrew Developer deroad is always contributing to either the PS3 scene or the PS Vita scene, he has released a few cool PS3 homebrew apps, contributed to the PS3/Vita Dev Wikis and has written some awesome tutorials, here is one of them, which he has recently updated:

In computing, a system call (aka Syscall) is how a program requests a service from an operating system’s kernel. This may include hardware related services (e.g. accessing the hard disk), creating and executing new processes, and communicating with integral kernel services (like scheduling). System calls provide the interface between a process and the operating system.
(Wikipedia – System Call)

With PSL1GHT we can call all the 989 Syscalls.
it’s really simple to call one, but you need to know how many parameters it needs; there are some syscalls that needs just 1 parameter, there are others that needs 8 parameters.

You can find the list of syscalls with the numbers of the parameters here: Ps3DevWiki Syscalls

Once you know the numbers of the parameter that you need (and the type of the parameters if they are written), you are ready to call one:

first thing add the following include:

#include <ppu-lv2.h>

Then you can call the syscall:

lv2syscallN(number_of_the_syscall, parameters… );
Change N with the number of the parameters, and put the number of the syscall that you need to call with the parameters.
some examples:

Shutdown the PS3:
1)Look into the dev wiki and you will see that the syscall to shutdown the PS3 is number 379 ( sys_sm_shutdown ) and it wants 4 parameters:
2)So now i can call the syscall with 4 parameters:

lv2syscall4(379,SHUTDOWN_PARAM,0,0,0);
now on the wiki it says, that the shutdown value is 0×1100 or 0×100, so i will write:
lv2syscall4(379,0×100,0,0,0)

now if i call this on a stupid homebrew like:
#include <ppu-lv2.h>
int main(){
lv2syscall4(379,0×100,0,0,0);
return 0;
}

it will really shutdown the ps3.

now if you are not sure to completely understood this, i will make another example:
Get current time:
now this example comes from the PSL1GHT SDK:

this is the function that get the current time on the ps3.

#include <ppu-lv2.h>
s32 sysGetCurrentTime(u64 *sec,u64 *nsec)
{
lv2syscall2(145,(u64)sec,(u64)nsec);
return_to_user_prog(s32);
}

now it uses 2 parameters so it calls lv2syscall2, then it needs the parameters that will get the value sec and nsec. now, in this function you can see that it ask the return of this syscall; to call any return of a determinate syscall, you can simply ask it by calling:

return_to_user_prog(type_of_the_variable);

you need to define the type of the returning variable. in that case it was s32 (aka signed int).
so it calls:
return_to_user_prog(s32);

if it was a normal integer, you had to call:
return_to_user_prog(int);

to compile it correctly you need to simply add the following flags:

-llv2

Visit the official source here:
Welcome on PS3 Tutorials

Also visit the developers blog here:
/dev/ram0