What is the difference between iso and secondary.

Hint: For giving common names for hydrocarbons we generally use prefixes like iso, sec, neo and tert. These prefixes basically describe the degree of carbon in that hydrocarbon. We use iso and sec prefixes for describing the carbon chain of hydrocarbons like whether they are linear or nonlinear.

Complete Answer:
The IUPAC nomenclature of the organic compounds is different from what we call them commonly. Such names are known as common names. There are certain rules for giving a common name for hydrocarbons. For this we generally use the prefix like iso, sec for secondary, neo and tert for tertiary. The prefix iso is used when the hydrocarbon has at most six carbon atoms in a chain and one carbon is present on the second last carbon atom of the chain.
For example: iso-butyl
\[{\text{ }}\begin{array}{*{20}{c}} {{\text{ C}}{{\text{H}}_3} - CH - CH - } \\
  | \\ {{\text{ }}C{H_3}} \end{array}\]
We will use secondary prefix or sec when the functional group is bonded to a secondary carbon atom. It is not used when the carbon chain is shorter than four. Therefore when the attachment of the alkyl group is on secondary carbon then we use prefix sec.
For example: sec-butyl
\[{\text{ }}\begin{array}{*{20}{c}} {{\text{ C}}{{\text{H}}_3} - CH - CH - } \\ {{\text{ }}|} \\ {{\text{ }}C{H_3}} \end{array}\]
Therefore we can see the difference between the iso and sec prefix of naming of hydrocarbons.

Note:
Iso can also be used when all the carbon atoms in the chain form a continuous long chain and one carbon is not a part of that chain which is attached at the second last carbon atom of the long chain. Sec prefix is used when the functional group present in the carbon chain is bonded to a secondary carbon atom.