On the 7th day of Pesach, the crossing of the Israelites at the Reed Sea is read. In Torah, the “Song at the Sea”  that the Israelites sing is written in a special calligraphic format. The same format is used for the text of the haftarah reading  as well as for the “Song of Deborah,”  which recounts the Israelites’ deliverance at another time.
“…in thirty lines, neither more nor less. The first line is to be full. The second line is to be divided into three written sections, with open space in between [the words]. The third line is to have two written sections, with white space in between. The fourth line is to be written as the second line, the fifth line as the third line, and so on. The visual effect is that of a brick wall.” 
The format was meant to give special importance to these sections, especially to the “Song at the Sea.” It’s also supposed to suggest the two walls of water through which the Israelites passed (think of the outer words as the walls, and the middle words as the Israelites walking through).
This format was mandated by the Masoretes (c. 9th century CE), based on the Talmud.  A Torah that does not contain this is not valid for public reading. Many chumashim recreate this format in print as well, including Plaut’s version for Reform Judaism.
We should keep in mind, though, that before the invention of the printing press, it was only “ba’alei korim” (Torah-readers) who saw this. At one time, this could include many members of the congregation. Gradually, fewer and fewer men could actually read Torah. No women read it at all until printed books appeared and until women began to read publicly from a sefer Torah in recent years. As fewer men were able to read from an actual sefer Torah, a “ba’al koreh” became a professional who was paid to invest the time necessary to prepare each week’s Torah reading. Still, because it appears in chumashim today, every congregant should be aware of why it’s written this way.
 Shemot/Ex. 15:1-19
 II Shmuel/Samuel 22:1-51
 Shoftim/Judges 5:1-31
 Kolatch, Alfred J.; This is the Torah; Jonathan David Publishers, NY, © 1988; p. 111-114
 Shabbat 103b