Pen and Ink can mean a lot of different things, and can be done with a lot of different materials depending on what you want to do. For this discussion, we'll assume that this is for good works, meant to be kept, framed, sold or given away. You can draw your Celtic knots with anything, but some things are more archival than others, meaning that they will last longer over time than others. Some pens (like ball points) will change color over time or fade, and some papers (like newsprint) will turn very yellow and brittle over time. This may not matter if you are only sketching, but if you want to make something that will look 5 years down the road like it did when you first painted it, you need to choose more archival materials. Most of these materials are available at art stores.
One of the most common types of pen used for pen and ink is a dip style pen and staff system. Much like in the old fashioned calligraphy pens, you purchase the nib for the pen and the body separately. The nibs are available in several different sizes, so choose one depending on the detail you want to do on your final piece. Some popular types are the Hawk and Crow quill. Some nibs are more flexible than others as well, meaning that a very flexible nib will allow you to vary the line thickness by varying the pressure you apply to the nib on the paper. The rigid nibs will give you a very even and consistent line as you draw.
You can also use pigment based pens like the Pigma by Sakura, or the Pigment Liner by Staedtler. These pens are waterproof when dry, and pigment based so they will not fade (unlike a dye based pen like a marker). They come in several different sizes depending on how finely detailed you are working, and also come in a few colors as well as basic black.
There is also a pen called the Technical pen, which is used a lot by draftsmen. These pens give you a wide range of pen sizes to choose from, and come with empty cartridges that you can fill with whatever colors you want. The only difficulty with these pens is they require quite a bit of maintenance and cleaning. Inside them is a very fine mechanism that you must be careful to not damage or let ink dry in. They are also quite costly as well (around $25-$30 Canadian each). They are a great pen if you think you will be doing a lot of detailed work all the time, and well worth learning to use, but if you only want a bit of detail here and there they can be a bit of a hassle for the beginner.
Colored inks are usually dye based, so you must be careful when you buy the ink to check with the sales staff or on the product information sheets to see if the ink you pick is lightfast. If it is not lightfast, over time the color will fade away or change color. Some dye inks are better than others, like the Ecoline by Talens, which resists fading to some extent. Winsor and Newton's Liquid Watercolors and their Drawing Inks work well too, but tend to fade some.
You can also get colored inks that are pigment based, which again means that most colors are more resistant to fading. Some colors, depending on the pigment, will still fade, but you have a better chance in these in finding some that won't. These will usually be rated on a separate chart or the product information sheet, and it will tell you which ones are lightfast, moderately lightfast, or not lightfast at all. A common version of the pigment based inks are actually a liquid acrylic. They have the same properties as a regular acrylic (large range of colors, waterproof when dry) except that they are very thin and runny, rather than pasty. Brands like Rowney and Rotring make a nice liquid acrylic, in both transparent and opaque colors. Be careful with the Rotring however, the company has discontinued them, and while you can still buy it from stores that have some left, you don't want to run out halfway through a project, or you'll be hard pressed to match the color again!
As far as a standard black ink goes, there are many versions you can choose from. Some good brands are Speedball and Pebeo, both make a nice, deep black ink that is also waterproof. You do not have to choose a waterproof ink of course, but I find that if I am using pen and ink to outline a knot, and then I want to fill the insides with color (like with a watercolor for example) then I want the outside lines to stay fixed and not run!
Papers for pen and ink are usually smooth, because of the detail involved. A very rough paper is difficult to make a nice straight line on - it tends to wiggle about a lot! If you are using a metal tipped nib (like a technical pen or a dip style pen) you want to stay with a smoother paper as well because the very fibre-y papers will jam up the nib and make it hard to work with.
The papers can come in different thicknesses, and the heavier ones are called bristol. The heavier papers are sometimes easier to work with because they don't get damaged as easily, but feel free to experiment with some of the different papers out there. There are a lot of neat handmade papers from around the world, with bits of leaves, petals, and bark in them to give neat effects. Always test out your ink on a little scrap of paper to see how much it will bleed. There's nothing worse than wrecking a piece of great paper because the ink you've picked bleeds on it! Different inks will react differently on different papers, so be sure to test before you start. Some stores will have samples that you can test on before you buy the paper as well.
Watercolor papers are another option, especially of you want to stain the background or use some watercolors on it later when you are done the outlines. Watercolor paper is very heavy, and is usually acid free and 100% rag (which means its made all out of cotton). These are good things in a paper! It means it has no acids in it that will cause it to turn yellow and break down over the years, and that it is made out of cotton rather than tree pulp, which can cause the paper to age prematurely as well. Look for these terms in any papers you buy. Another term is neutral pH, which is not quite as good as acid free, but is better than nothing. It means that rather than having no acids in the paper at all, there were acids present but they were neutralized. Over time the acids may act on the paper again, which is why this situation is still not as good as acid free.