Definitions of retrotransposon on the Web:

A class of genetic elements that includes retroviruses and transposons that have an intermediate RNA stage. A transposon that was created by reverse transcription of an RNA molecule.
helios.bto.ed.ac.uk/bto/glossary/qr.htm

Retrotransposons are mobile genetic elements and are ubiquitous in the genomes of many eukaryotic organisms. They are particularly abundant in plants, where they are often a principal component of nuclear DNA. In maize 50-80%, and in wheat up to 90%, of the genome is made up of retrotransposons.
en.wikipedia.org/wiki/Retrotransposon

This is what happens during a normal cycle:
DNA ----> RNA --->protein

Transposons are DNA sequences that jump from one place to another on the DNA strand. This causes mutations, deletions, etc. (DNA ----> RNA --->no protein = no function, or altered protein = no function or other function, etc.)
Retrotransposons are sequences of DNA that enter the cell, are transcribed into RNA, then retrotranscribed into DNA, and then inserted into the host DNA. So :
DNA ----> RNA ---->DNA ---->RNA ----> other species' protein that is expressed in the host, where most likely it has caused a mutation where it was inserted.

Both jump from one location to another, the difference is that the first occurs within the cell itself while the other comes from outside. Also the second has the enzymes for retrotranscription.

Below is the section on retrotransposons from Wikipedia:

Biological activity
The retrotransposons replicative mode of transposition increases the copy numbers of elements rapidly and thereby greatly increasing plant genome size. Like DNA transposable elements, they can induce mutations by inserting near or within genes. Furthermore, retrotransposon induced mutations are relatively stable; because the sequence at the insertion site is retained as they transpose via replication mechanism.

Retrotransposons copy themselves to RNA and then, via reverse transcriptase, back to DNA. Transposition and survival of retrotransposons within the host genome are possibly regulated both by retrotransposon- and host-encoded factors, to avoid deleterious effects on host and retrotransposon as well, in a relationship that has existed for many millions of years between retrotransposons and their plant hosts. The understanding of how retrotransposons and their hosts genomes have co-evolved mechanisms to regulate transposition, insertion specificities, and mutational outcomes in order to optimize each other's survival is still in its infancy.

Types of retrotransposons
Retrotransposons belong to class I type of mobile elements, consists of two sub-types, the long terminal repeat (LTR) and the non-LTR retrotransposons. The LTR retrotransposons have direct LTRs that range from ~100 bp to over 5 kb in size. LTR retrotransposons are further sub-classified into the Ty1-copia and the Ty3-gypsy groups based on both their degree of sequence similarity and the order of encoded gene products. Ty1-copia and Ty3-gypsy groups of retrotransposons are commonly found in high copy number (up to a few million copies per haploid nucleus) in plants with large genomes. Ty1-copia retrotransposons are abundant in species ranging from single-cell algae to bryophytes, gymnosperms, and angiosperms. Ty3-gypsy retrotransposons are also widely distributed, including both gymnosperms and angiosperms. LTR retrotransposons make up approximately 8% of the human genome.

The non-LTR retrotransposons, consists of two sub-types, long interspersed nuclear elements (LINEs) and short interspersed nuclear elements (SINEs). They can also be found in high copy numbers (up to 250,000) in the plant species.

LINES (long interspersed elements) are long DNA sequences that represent reverse-transcribed RNA molecules originally transcribed by RNA polymerase II into mRNA (messenger RNA to be translated into protein on ribosomes). Also called pseudogenes, they do not contain introns or promoters, but can code for reverse transcriptase or integrase, enabling them to copy both themselves and other, noncoding LINES. Because LINES move by copying themselves (instead of moving, like transposons do), they enlarge the genome. The human genome, for example, contains about 500,000 LINES, which is roughly 21% of the genome. LINES are used to generate genetic fingerprints.

SINES (short interspersed elements) are short DNA sequences that represent reverse-transcribed RNA molecules originally transcribed by RNA polymerase III into tRNA, rRNA, and other small nuclear RNAs. SINEs do not encode a functional reverse transcriptase protein and rely on other mobile elements for transposition. The most common SINES are called Alu sequences. Alu elements are about 300 base pairs long, do not contain any coding sequences, and can be recognized by the restriction enzyme AluI (thus the name). With about 1 million copies, they make up about 11% of the human genome. Both LINES and SINES are also called "selfish DNA" or "junk DNA", because they do not serve any known purpose. SINEs make up 13.5% of the human genome.

Several viruses, like HIV-1 or HTLV-1 behave like retroposons and contain both reverse transcriptase and integrase, the retroposon equivalent of transposase.

Important points are that retrotransposons are present in the DNA of organisms and that unlike transposons they can increase in copy number within the DNA.

The origin of retroelements (retrotransposons and retroviruses) is still under investigation. Apparently RNA is considered to have evolved before DNA and there are questions of which came first in terms of retroelements and whether all retrotransposons are similar in origin and evolution.

Some articles treat the term retransposon as a synonym for retrotransposon.

(My apologies if this post appears twice. I thought I had posted it but it did not appear)