Until recently (and I have lived in Japan a long time), this word was completely unknown to me. The word comes from two words, yoseru, meaning to bunch up together, and moji, meaning letters. Together it means a kind of big bold calligraphy with thick strokes and narrow spaces between the strokes. This kind of writing is sometimes called flyer or poster letters, and I believe that there are actually several genres, though I would not be able to tell the difference. There are several places where you can see this type of lettering. I think the most well known location is for the schedules of the sumo bouts. It is also used for theatrical performances, particularly comic story telling, and is also used on various kinds of menu, for advertising the types of fish etc.
Last year I was fortunate enough to meet Mr. Umon Tachibana. He is a yosemoji artist. He has been sent by the Japanese government as a cultural ambassador, to teach yosemoji to people in different countries, and he went to the UK, where he demonstrated his art at the Japan Society North West. He explained to me that when writing for theatrical performances, the white spaces in the letters represent empty seats in the theater, so that it is unlucky to have big white spaces, and that is why the strokes have to huddle together, to minimize the white spaces in the letters. Umon Tachibana recenty organized a rakugo event, which I attended, and all the signs, of course, were written by him. In rakugo, the messenger, or storyteller sits on a cushion, and tells amusing stories, and beside them the name of the storyteller is posted, on a long piece of washi paper.
Here are the links to his site, where you can see yosemoji. The Japanese site has more photographs.
Recently he has also been designing small, cotton hand towels in conjunction with a stencil artist. The results are like popular Edo period art. Very simple, strong and typical of Tokyo. The towel here is an image of a menu and all the dishes, ready to eat a traditional Japanese meal.
Mr. Umon Tachibana is a very creative and original artist and a delightful and fun loving person, who I am very glad to know. I hope you will look at his website.