Pili are proteic retractile filaments up to 20 micrometer long that protrude from gram-negative bacteria. Some RNA and DNA bacteriophages use pili to attach to the host cell.
There are many types of pili and each bacterial virus binds specifically to a precise type. Some filamentous viruses like bacteriophage fd adsorb onto the tip of F-pili and enter the cell at the pili basal pore. Other filamentous phages can target I-pili (phage If1), N-pili (phage IKe), or type IV pili (e.g. bacteriophages Pf1 and Pf3) 
.
Icosahedral virus usually adsorb onto the sides the pili.
Viral attachment to host cell pilus (kw:KW-1175)
Identification and specificity of pilus adsorption proteins of filamentous bacteriophages infecting Pseudomonas aeruginosa.
Holland SJ, Sanz C, Perham RN
Virology. 2006 Feb 20;345(2):540-8
Holland SJ, Sanz C, Perham RN
Virology. 2006 Feb 20;345(2):540-8
Pseudomonas aeruginosa transposable bacteriophages D3112 and B3 require pili and surface growth for adsorption
C Roncero, A Darzins, M J Casadaban
J. Bacteriol. April 1990; 172: 1899?1904
C Roncero, A Darzins, M J Casadaban
J. Bacteriol. April 1990; 172: 1899?1904
Matching UniProtKB/Swiss-Prot entries
(all links/actions below point to uniprot.org website)17 entries grouped by protein
9 entries
Attachment protein G3P (Gene 3 protein) (G3P) (Minor coat protein)
4 entries
Maturation protein A (MP) (Assembly protein) (A protein)
3 entries
Maturation protein A2 (MP) (A2) (Assembly protein) (A protein)
1 entry