Composing and Decomposing Numbers
349 
is made up of 

Composing
So 349 is composed of 3 hundreds, 4 tens and 9 units, in other words:
300 + 40 + 9 ⇒ 349 
"Composing" 
Example: Compose 2 hundreds, 7 tens and 9 units.
200 + 70 + 9 = 279
Decomposing
Decomposing is when we break the number apart
349 ⇒ 300 + 40 + 9 
"Decomposing" 
Example: Decompose 3,456
3,456 = 3,000 + 400 + 50 + 6
Standard and Expanded Form
Some people call the two different forms "Standard" and "Expanded":
349  300 + 40 + 9 
Standard Form  Expanded Form 
Example: Put 925 into Expanded Form
925 = 900 + 20 + 5
Decimal Numbers
We can also compose and decompose decimal numbers:
37.2 ⇒ 30 + 7 + 0.2 
"Decomposing" 
4 + 0.7 + 0.08 ⇒ 4.78 
"Composing" 
Example: Decompose 1.35
1.35 = 1 + 0.3 + 0.05