Long Terminal Repeats (LTRs) are features of LTR retrotransposons + double-stranded proviral DNA (result of reverse transcription of single-stranded retroviral RNA).
Importantly, LTRs are direct repeats which flank the transposed coding regions + themselves are flanked by Target Site Duplications (TSDs).
Their structure comprises three domains: U3, R + U5. U3 and U5 contain signals for synthesis + processing of the retroviral genome/retrotransposon; R, usually 10 to 230bp in length, "is necessary for DNA strand transfer during reverse transcription". 
It should be noted that extremely high sequence conservation has been observed in the two bases at each of the 3' + 5' termini of all human LTRs. 
In wheat retrotransposon Wis 2-1A, the length of the LTR is unusually long: 1755bp . Interestingly, approximately 45% of these LTR structures are comprised of hairpins, caused by inverted repeats (gapped palindromes).
Why do we care about LTRs? Here is one incredible reason: LTR-targeted mutation of the HIV-1 provirus using CRISPR/Cas9 has been shown  to reduce LTR-driven expression of the virus!
2. http://gydb.org/index.php/LTRs_and_TIRs [great article explaining the difference between LTRs and Terminal Inverted Repeats (TIRs)]
3. Lucas, H., Moore, G., Murphy, G. and Flavell, R.B., 1992. Inverted repeats in the long-terminal repeats of the wheat retrotransposon Wis 2-1A. Molecular biology and evolution, 9(4), pp.716-728.
4. Ebina, H., Misawa, N., Kanemura, Y. and Koyanagi, Y., 2013. Harnessing the CRISPR/Cas9 system to disrupt latent HIV-1 provirus. Scientific reports, 3.
5. Shah, S., Alexaki, A., Pirrone, V., Dahiya, S., Nonnemacher, M.R. and Wigdahl, B., 2014. Functional properties of the HIV-1 long terminal repeat containing single-nucleotide polymorphisms in Sp site III and CCAAT/enhancer binding protein site I. Virology journal, 11(1), p.92.
6. Howe, M. and Berg, D., 1989. Mobile DNA. Amer Soc Microbiol, Washington DC. [see chapter 3]
7. Lee, I. and Harshey, R.M., 2003. Patterns of sequence conservation at termini of long terminal repeat (LTR) retrotransposons and DNA transposons in the human genome: lessons from phage Mu. Nucleic acids research, 31(15), pp.4531-4540.
8. Jeeninga, R.E., Hoogenkamp, M., Armand-Ugon, M., de Baar, M., Verhoef, K. and Berkhout, B.E.N., 2000. Functional differences between the long terminal repeat transcriptional promoters of human immunodeficiency virus type 1 subtypes A through G. Journal of virology, 74(8), pp.3740-3751.