Category Map Example

//Assume Asset object has name, lookup to user and picklist called Category
//Store results in a map
Map <String, List<String> > assetmap = new Map <String, List<String> >();
//Loop through assets
for (Asset__c tempasset : [Select Name, Category__c FROM Asset__c WHERE User__c = '0053000000AD9YT'] ) {
    List<String> tempassetlist;
  //if map already contains category, add to existing list
  If (assetmap.containskey(tempasset.Category__c) ){
        tempassetlist = assetmap.get(tempasset.Category__c);
    }
    //or create empty list
    else {
        tempassetlist = new List<String> ();
    }
    //Update list
    tempassetlist.add(tempasset.Name);
    //update map
    assetmap.put (tempasset.Category__c, tempassetlist);
}
//see results
system.debug(assetmap);