parse html:what is the meaning of "//"?

[alias]

code1:
import lxml.html
import urllib
down='http://finance.yahoo.com/q/op?s=C+Options'
content=urllib.urlopen(down).read()
root=lxml.html.document_fromstring(content)
table = root.xpath("//table[@class='yfnc_mod_table_title1']")[0]
tds=table.xpath("tr[@valign='top']//td")
for  td  in tds:
    print  td.text_content()

what i get is :
Call Options
Expire at close Friday, September 16, 2011
these are waht i want.

code2
import lxml.html
 import urllib
 down='http://finance.yahoo.com/q/op?s=C+Options'
 content=urllib.urlopen(down).read()
 root=lxml.html.document_fromstring(content)
 table = root.xpath("//table[@class='yfnc_mod_table_title1']")[0]
 tds=table.xpath("//tr[@valign='top']//td")
 for  td  in tds:
     print  td.text_content()

what i get is :
N/A
N/A
2
114
48.00
C110917P00048000
16.75
 0.00
N/A
N/A
0
23
50.00
C110917P00050000
23.16
 0.00
N/A
N/A
115
2,411
   
   
   
Highlighted options are in-the-money.
(omit  something)
there is only one difference between   code1 and code2  :
in code1 is :   tds=table.xpath("tr[@valign='top']//td")
in code2 is:   tds=table.xpath("//tr[@valign='top']//td")

i want to know  why  the  "//"  make output  different?
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