.YDK files are Yu-Gi-Oh! card lists generated by YGOPro containing cards serial numbers. YDKList parses YDK files and searches on third-party databases (http://www.yugiohprices.com/ and http://yugioh.wikia.com/) for more information (including price on online marketplaces) about each card serial number.
You can use this YDK file as example:
#created by ygopro2
#main
89631139
89631139
89631139
38517737
38517737
38517737
45467446
45467446
72855441
71039903
71039903
71039903
14235211
45644898
45644898
79814787
97268402
88241506
88241506
88241506
38120068
38120068
38120068
39701395
39701395
39701395
41620959
41620959
41620959
48800175
48800175
48800175
6853254
54447022
2295440
51405049
51405049
24094653
87025064
87025064
50371210
24382602
24382602
29401950
#extra
23995346
56532353
56532353
59822133
59822133
59822133
40908371
40908371
40908371
66547759
10678778
!side