There are somewhere around 230 strings in a piano. They range from a speaking length of only 2″ to 7 feet or more. In the low bass there is only one string per note. Then in the upper bass, 2 strings per note. In the middle and treble there are 3 strings per note. This is to balance out the loudness of each area. The upper strings are plain steel wire. Although they all look alike, there are many sizes of plain wire that are only .001 inch different. In the bass section, the strings have copper windings around them to make them heavier so they will vibrate more slowly. The lowest notes can have 2 layers of copper to make them large without making them too stiff.
Tuning a piano involves adjusting every string. I first get one of the strings in the right relation to notes around it, and then tune the unison to match the other two strings to the one already tuned.
Piano wire is probably the strongest material in your house. It is very difficult to make it hard and strong enough without it becoming brittle and breaking easily.
Strings on a piano break sometimes just as they do on other string instruments. One repair that can be done is to splice the string back together with a tuners knot. This is often a better repair than replacing the string because it can be done immediately. Bass strings have to be made to order for each piano so that takes time. Also a spliced string settles back down to tuning stability quicker than a new string does.
I know it seems strange to picture tying a knot in piano wire but this can be done and creates a good repair.
Here are some pictures of a set of very worn hammers and some new ones. The worn ones are very grooved and you can see a hammer where the wooden molding is showing through where the felt is completely gone. The new hammers show a nice rounded shape.
What sound do worn hammers make? When played loudly the flat surface of the hammer imparts a square shape into the string and brings out a lot of high harmonics which make for a bright or brittle sound. When played softly the grooves can actually make the hammer stick on the string for a short moment and make it dull sounding.
Hammers can be resurfaced by filing off layers of the felt to restore a smooth striking surface and a rounded shape. However the hammers in this picture would not improve a lot from filing them because there is so little felt left. These hammers need to be replaced.
Here we have an example of a split bass bridge. This will create a buzzing noise as the bridge pins rattle against the strings. The strings are supposed to be staggered as they go through the pins Notice the lines on the bridge that show where the strings used to be, and how the pins in the upper row have moved over to the right. To repair this requires making a new bridge cap.
When looking at a used piano, checking the state of the bridges is important. Repair of this is more expensive.