When we say about the similarities between retroviruses and retrotransposons, both retroviruses and retrotransposons insert themselves into the genome of the host as double stranded DNA (Deoxyribonucleic acid) copies of a RNA (Ribonucleic acid) intermediate.
Both express the enzyme called reverse transcriptase that can synthesize DNA from a RNA template.
The structure of retroviruses and transposes are the similar in having long terminal repeats at the ends.
Now we can say about the differences between them.
Although the retrotransposons carry genes related to viral genes gag and pol required for RNA processing and DNA synthesis, they do not carry the env gene required for the generation of viral particle and cannot leave the cell.
Both are assumed to be evolutionarily related by the conservation of the structure and replication of the elements.
But we cannot agree with the hypothesis that retroviruses evolved from retrotranspososns.