The verbs "unravel" and "ravel" mean the same thing. Personally, however:

1) I use the word "unravel" to signify a DELIBERATE act of unstitching in my work.

Example: "I unraveled two rows of knitting because I noted a wrong stitch two rows below my current row."

2) I use the word "ravel" to signify ACCIDENTAL unstitching in a knitted work.

Example: "After soccer practice he came to me and showed me the raveled edge of his sweater sleeve."