Two thirds represents the largest part of the whole. The smaller the denominator the larger each unit is.
Fractions as Part-Whole concept is the most commonly used example to teach fractions to children. A fraction represents one or more parts of a geometrical figure which has been divided into some equally-sized pieces.
Probably the most commonly used fraction is 1/2, which is one-half. When you cut a cake into two pieces and take one for yourself, you get 1/2 of the cake.
We call the top number the Numerator, it is the number of parts we have. We call the bottom number the Denominator, it is the number of parts the whole is divided into (Numerator / Denominator).
You just have to remember those names! (If you forget just think "Down"-ominator).
Know more about the basics of fraction at Dummies.com and MathisFun.com.