There's a few useful things you can do with SD objects we'll list a few here:

• If you just want to check if a file exists, use SD.exists("filename.txt") which will return true or false.
• You can delete a file by calling SD.remove("unwanted.txt") - be careful! This will really delete it, and there's no 'trash can' to pull it out of.
• You can create a subdirectory by calling SD.mkdir("/mynewdir") handy when you want to stuff files in a location. Nothing happens if it already exists but you can always call SD.exists() above first.

Also, there's a few useful things you can do with File objects:

• You can seek() on a file. This will move the reading/writing pointer to a new location. For example seek(0) will take you to the beginning of the file, which can be very handy!
• Likewise you can call position() which will tell you where you are in the file.
• If you want to know the size of a file, call size() to get the number of bytes in the file.
• Directories/folders are special files, you can determine if a file is a directory by calling isDirectory()
• Once you have a directory, you can start going through all the files in the directory by calling openNextFile()
• You may end up with needing to know the name of a file, say if you called openNextFile() on a directory. In this case, call name() which will return a pointer to the 8.3-formatted character array you can directly Serial.print() if you want.