There are different ticket prices for the same seat, depending on when you travel, how far you book in advance, and whether you have a railcard. There are also some secret sauce ingredients that I don’t completely understand. Here are the basics:
Anytime Tickets: valid any time, any day, any train, any operator. An Anytime Single allows you to travel within 2 days of the date shown on your ticket. With Anytime Returns the outward part is valid for 5 days from the date on your ticket, and the return for 1 month. Anytime Day tickets are only valid on the date shown. Anytime tickets are the most expensive way to travel but also the most convenient if you don’t/can’t have a set schedule.
Off-Peak and Super Off-Peak Tickets: cheaper, but don’t allow you to travel during peak times (rush hour for example) and/or on certain days or routes. In general, Off-Peak hours begin at 09:30 Monday to Friday in cities and large towns, and at 09:00 everywhere else. Weekends and bank holidays are Off-Peak all day.
Where there is more than one Off-Peak fare for your journey, the cheaper fare – which of course comes with more restrictions – will be the Super Off-Peak. Both options are valid only on the date shown on your ticket, with returns valid for one month from the ticket date.
Advance Tickets: must be bought in advance and are only valid on the date and train specified. Can be the cheapest way to travel but there is limited availability at each price level. As with airline fares, the price can rise closer to the departure. Advance tickets are sold subject to availability and on a first come, first served basis. You can usually buy an Advance ticket up to 12 weeks ahead, but on some routes they are available until the day of travel, even up to 10 minutes before departure.
Advance tickets are priced as one-way fares, enabling you to combine the cheapest fare for your outward journey with the cheapest fare for your return leg. Book early and hunt around the different departures to find the cheapest fare. Advance tickets must be booked by 18:00 the day before travel, although they're often still available until 23:59.
As a general rule, the earlier you buy a ticket, the better the deal you can get. If you know the exact date and time you want to travel, then an Advance may be best. For more flexibility, Off-Peak and Super Off-Peak and Anytime will be a better choice, as you are not limited to a specific date and/or train.