An electron is a quantum particle. Like all quanta, we do not know where it is (location) until we locate it. By a strange quirk of science, for reasons we do not understand, the equations of Wave Mechanics can be used to calculate the probability of locating an quantum at a given location. An equation to do that is called a "Wave Function." Wev can use the Wave Functions of the electrons in an atom to draw a volume that has a high probability of containing the electrons. These volumes are called "Orbitals."
The orbitals are NOT neat circles because electrons are not little balls that orbit like planets. Here are pictures of the various orbitals.
http://en.wikipedia.org/wiki/Atomic_orbital
http://images.search.yahoo.com/images/view?back=http%3A%2F%2Fsearch.yahoo.com%2Fsearch%3Fei%3DUTF-8%26p%3Dorbitals&w=160&h=99&imgurl=www.bing.com%2Fimages%2Fsearch%3Fq%3Dorbitals%23focal%3Dde278cdcd5b2ebd6401f64108f32f395%26furl%3Dhttp%253a%252f%252farchivecrawl.com%252fwp-content%252fuploads%252f2010%252f06%252forbitals.gif&size=&name=search&rcurl=http%3A%2F%2Fwww.bing.com%2Fimages%2Fsearch%3Fq%3Dorbitals%23focal%3Dde278cdcd5b2ebd6401f64108f32f395%26furl%3Dhttp%253a%252f%252farchivecrawl.com%252fwp-content%252fuploads%252f2010%252f06%252forbitals.gif&rurl=http%3A%2F%2Fwww.bing.com%2Fimages%2Fsearch%3Fq%3Dorbitals%23focal%3Dde278cdcd5b2ebd6401f64108f32f395%26furl%3Dhttp%253a%252f%252farchivecrawl.com%252fwp-content%252fuploads%252f2010%252f06%252forbitals.gif&p=orbitals&type=&no=3&tt=114&oid=http%3A%2F%2Fts4.mm.bing.net%2Fimages%2Fthumbnail.aspx%3Fq%3D4634532299342039%26id%3D5135f5200a23b7feb1d7aeca418defc6&tit=orbitals&sigr=158pde359&sigi=151hof2sb&sigb=11i3pu3oh
Those are your "Boundary Surface Diagrams."
A quantum like an electron is described by numbers. One of the numbers is its "Spin" Spin is important because electrons of the same spin cannot be in the same orbital.
http://en.wikipedia.org/wiki/Pauli_exclusion_principle