Fallback: As the name is self explanatory to use something as an alternative if the primary thing does not work well. In xslt the fallback tag works as an alternative tag if any given tag does not work or is not supported by xslt processor. 
Note: This fallback will always work in not supported tags, so run time exception will not occur.
Data.xml

1 2 3 4 5 6 7 8 9 10 11 12 13 14<?xml version="1.0"?><?xml-stylesheet type="text/xsl" href="XSL.xsl"?><records><record><name>Adam Barr</name><address>222 Cherry</address><phone>555-797-2355</phone></record><record><name>Jeff Adell</name><address>730 Elm</address><phone>555-797-5555</phone></record></records> XSL.xsl
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46<?xml version="1.0"?><…