difference between glyceraldehyde-3-phosphate and 3-phosphoglycerate
The difference between the two molecules is highlighted in red.



The functional group in 3-phosphoglycerate is a carboxylic acid. That in glyceraldehyde-3-phosphate is an aldehyde.

Sugars have the general formula $\ce{C_$n$H_{$2n$}O_$n$}$. To accomplish this specific ratio of carbon to hydrogen to oxygen, the requirement is that one of the carbons must have a carbonyl group (i.e. forms an aldehyde or ketone), whereas the remainder of the carbons have one hydroxyl group each. For example, the open-chain structure of glucose is:



The carbonyl group is in red and the hydroxyl groups are in blue.

Therefore, glyceraldehyde-3-phosphate is simply a phosphorylated sugar: C-1 is the aldehydic carbon and C-2 and C-3 have hydroxyl groups, the latter of which is phosphorylated.

3-Phosphoglycerate is one oxidation state too high.