WMV is really just another name for ASF. That format, like many multimedia formats, contains its data in pieces of various types. Some formats call these packets, some call them segments. ASF, like RIFF before it, calls them chunks. There are several types of chunks, the main one being the data chunk, which holds the encoded streams. There may be several data chunks in the file, mixed in with other chunk types.
The other chunks can contain metadata, DRM info, text data and so on. WMV includes one called (I believe) the index chunk which holds information about where the data chunks are located in the file.
If the index chunk is corrupted or missing the file can usually be played but perhaps not seeked, or may seek randomly into the middle of GOPs. Programs like ASFTools and Steeper attempt to re-index the file by locating the data chunks and building a new index chunk if they can't locate a misplaced one.
Chunks of various types begin with unique identifying headers -- byte sequences called tags -- that can be found by scanning the file. This allows a repair program to potentially reconstruct a minimally playable file from a partially corrupt one by examining the full set of chunks.