salesforce SOQL : query to fetch all the fields on the entity

You have to specify the fields, if you want to build something dynamic the describeSObject call returns the metadata about all the fields for an object, so you can build the query from that.

Create a map like this:

Map<String, Schema.SObjectField> fldObjMap = schema.SObjectType.Account.fields.getMap();
List<Schema.SObjectField> fldObjMapValues = fldObjMap.values();

Then you can iterate through fldObjMapValues to create a SOQL query string:

String theQuery = 'SELECT ';
for(Schema.SObjectField s : fldObjMapValues)
{
   String theLabel = s.getDescribe().getLabel(); // Perhaps store this in another map
   String theName = s.getDescribe().getName();
   String theType = s.getDescribe().getType(); // Perhaps store this in another map

   // Continue building your dynamic query string
   theQuery += theName + ',';
}

// Trim last comma
theQuery = theQuery.subString(0, theQuery.length() - 1);

// Finalize query string
theQuery += ' FROM Account WHERE ... AND ... LIMIT ...';

// Make your dynamic call
Account[] accounts = Database.query(theQuery);

superfell is correct, there is no way to directly do a SELECT *. However, this little code recipe will work (well, I haven't tested it but I think it looks ok). Understandably Force.com wants a multi-tenant architecture where resources are only provisioned as explicitly needed - not easily by doing SELECT * when usually only a subset of fields are actually needed.