The tiny little dot over the lowercase i and j has a name: tittle.
The tittle (Latin: titulus) originally a larger mark than it is today, served as a way to differentiate the i and j and keep them from getting lost in a sea of letters. In today's fonts, the shape of the tittle can serve as an identifying element in some typefaces. It may be a circular dot, a square, a diamond, or some other shape.
- What is the name of the dot over "j" and "i," and why do we use them? provides more history on this diacritic dot.
- Merriam-Webster Diotionary: tittle also shows examples of the use of the word tittle when it means a very small part.
The phrase "dot your i's and cross your t's" means to pay attention to details or to be very clear in your writing (or speaking). Today's typefaces automatically add the dots (tittles) on the i and cross strokes on the t but in handwritten notes it is easy to omit them, especially when in a hurry. See English for Students and Yahoo! Answers