What is the difference between isobutyl and sec-butyl?

Hint: There are a number of organic groups which act as substituent in writing IUPAC name of the compound.The purpose of the IUPAC system of nomenclature is to establish an international standard of naming compounds to facilitate communication. The goal of the system is to give each structure a unique and unambiguous name, and to correlate each name with a unique and unambiguous structure.

Complete answer:
Isobutyl and sec-butyl are alkyl radicals which are produced from their parent compound butane.
Butane is represented by the general formula RH where R is called an alkyl group or radical and they are represented as \[{C_n}{H_{2n + 1}}\].
The alkyl group contains only one hydrogen atom less than the alkane. Hence, alkyl radical of butane has molecular formula $\left( {C{H_3}C{H_2}C{H_2}C{H_2} - } \right)$
As we know the basic structure of butane is –

In the molecular structure of butane, we see that there are four secondary hydrogen atoms which are equivalent in nature. Replacing any one of these equivalent hydrogen by the methyl group from the parent structure of butane will lead to formation of a structure which is commonly known as isobutyl.

Hence, the total number of carbons in isobutyl remains the same, only the position of the methyl group is changed.
Similarly, removal of one of the secondary hydrogens from the parent structure will lead to the formation of a structure commonly known as sec-butyl.

Hence, the total number of carbons in sec-butyl remains the same, the position of all the carbon remains the same, only the number of hydrogens is reduced by one.
Hence, the basic difference between isobutyl and sec-butyl is the arrangement of carbon atoms to the parent molecule.

Note:
Both compounds namely isobutyl and sec-butyl can react with the same reagent to form different products. For example, on reacting with aqueous sodium hydroxide isobutyl form $2 - $ methyl propane $ - 1 - $ ol, while sec-butyl form butan $ - 2 - $ ol in the presence of the same reagent.