Flour for bread can be made of many different cereals. In the UK the primary cereal used is wheat, and the flour is usually denoted as ‘strong’, meaning that the flour contains a high proportion of gluten, which is necessary to enable a good ‘rise’. This results in a lighter, airy loaf.
Strong flour tends to be more expensive than the softer flours, mostly because it requires imported wheat to increase the gluten content.
Strong brown flours are normally sold as whole meal (including all the bran in the wheat) or wheat meal (where the amount of bran is reduced). You can also buy various other bread flours, including ones that have seeds, malt or other ingredients already incorporated.
White flour is by far the easiest to learn to make bread with, and I recommend for your first few loaves that you stick with that.
It is not the only white flour available in the UK, but I always try to buy Allinsons strong white flour. This is not because it is necessarily any better, but I had my best early experiences in bread making with this particular brand (and also their brand of yeast) and so stick with it.