Difference between isopropyl and isobutyl
The IUPAC Organic Chemistry Nomenclature does not explicitly define the "iso-" prefix meaning, as it is not used in the systematic naming. However, it is mentioned on several places when listing acceptable semi-systematic names. In the section A-2 (1979 Blue book, some also in 1993 Recommendations, section R-9.1, table 19a) those are (not mentioning neo- compounds)


isobutane (CH3)2CH−CH3
isopentane (CH3)2CH−CH2−CH3
isohexane (CH3)2CH−CH2−CH2−CH3


And in the section A-2.25 (some also in R-9.1, table 19b), where it lists acceptable "isoalkyl group" names:


isopropyl (CH3)2CH−
isobutyl (CH3)2CH−CH2−
isopentyl (CH3)2CH−CH2−CH2−
isohexyl (CH3)2CH−CH2−CH2−CH2−


Some related isomeric alkyls mentioned in this section are


sec-butyl CH3−CH2−CH(CH3)−
tert-butyl CH3−CH(CH3)2−
tert-pentyl CH3−CH2−CH(CH3)2−


There are also some other, unsaturated "isoalkyl" group names mentioned: 


isopropenyl CH2=C(CH3)−
isopropylidene (CH3)2C=


From the above we can induce that "iso-" prefix, when related to alkanes or alkyl groups, it means the simplest (n-)alkane/alkyl isomer branched at the end of the carbon chain.

Isopropyl happens to be secondary (sec) alkyl (there's lack of carbon atoms to create primary alkyl isomeric to the n-propyl), all higher isoalkyls are primary (no branching at the alkyl "attachment").