The music staff is the set of lines we read music on. What this staff looks like depends on your instrument. Some have just one set of lines, and some have two.
This is the most basic one. It is simply five lines that will tell us some pitch. The notes go either on the lines or the spaces between the lines. We can't tell what these notes are yet because we don't have a clef to tell us which notes are which. So for this lesson, we'll ignore notes and just look at the staff. When you're ready you can check out clefs and learn more about notes.
When we see one "Treble" staff together with one "Bass" staff, we call it a Grand Staff. Here is a grand staff with some notes added.
In written music, we group the notes into measures or bars. These barlines show how the measures are divided.
We can't tell how many notes to put into a measure yet because we don't have a Time signature. When you're ready to learn more about measures, take a look at that.
A staff is like a line of written text in a book: it goes from left to right, and when you get to the end of the line, it starts again on the next line.
Sometimes an instrument such as the piano, or a group of instruments played together such as harmonizing voices, will have multiple staves per line. In this case, each set is called a staff system. When looking at a piece of music, you might see multiple instruments in one system.
That's pretty much all there is to staves and staff systems. They are just the lines on which we write music. We'll learn more on writing in other lessons. Check out Clefs to get started.