A nice explanation given in this article.

http://www.evolutionsbiologie.uni-konstanz.de/gonpdf/8.pdf

. Very basic question on genetics. Why don't you read some standard books on basiceukaryotic genetics. You may even find some good reviews on the net if search for. However, you must make the effort to read it.

In all organisms, messenger-directed protein synthesis is catalyzed by ribosomes. All eukaryotic and prokaryotic ribosomes consist of ribosomal ribonucleic acids (rRNAs) and ribosomal proteins. At the beginning, rRNAs were believed to play only a structural role as a scaffold for ribosomal proteins, but recently they were shown to also be the key catalytic components of peptidyl transferase.

5S rRNA has a length of ∼120 nt and a molecular weight ∼40 kDa. It is found in virtually all ribosomes with the exception of mitochondria of some fungi, higher animals and most protists. In prokaryotic ribosomes it was shown to bind several ribosomal proteins: L5, L18 and L25 but in eukaryotes, 5S rRNA binds only ribosomal protein L5 (4).

For many years studies on 5S rRNA have been focused on a sequence analysis and its application to the problems of molecular evolution. The size and ubiquity of 5S rRNA that enabled RNA sequencing using direct methods, made it an ideal candidate for a molecular phylogenetic marker. This led to fast accumulation of the sequence data from a variety of organisms belonging to diverse taxonomic groups. Despite many years of research, very little has been known about details of its structure and function.

Recently, the crystal structure of the large subunit of the halophile archaea Haloarcula marismortui at 2.4 Å resolution has been solved. It gave us an insight into the tertiary structure of rRNAs, including 5S rRNA, and their interactions with other components of the ribosome.

In simple to estimate phylogenetic relation ships among species.

Ganapathi uploaded a good article and Tarakeswar too mentioned the points.