gistfile1.cls

//Kick off with
Database.executeBatch(new batchUpdate(),50);


//Class like this
global class batchUpdate implements Database.Batchable<sObject> {
    // This method returns a SOQL query locator containing the records  
    // to be iterated
    global Database.QueryLocator start(Database.BatchableContext BC){
        return Database.getQueryLocator([SELECT Id,Something__c FROM CustomObj__c]);
    }
    // The executeBatch method is called for each chunk of records returned from start.   
    global void execute(Database.BatchableContext BC, List<CustomObj__c> scope){
        for(CustomObj__c obj : scope){
            obj.Something__c = 'wat';
        }
        update scope;
    }
    global void finish(Database.BatchableContext BC){  

    }
}