There are many different seed mixtures available to gardeners, which can sometimes be confusing. However, always think about what you need from your lawn, in terms of durability and maintenance, and keep this in mind when looking at seed mixtures.
In general, seed mixtures cover these main areas:
General-purpose lawn: This is often a mix of hard-wearing grasses, and is suitable for areas that receive a lot of wear and tear, perhaps from children or pets, or for areas of high traffic. These mixtures will often be a blend of perennial ryegrass, tall fescue, red fescues and browntop. The lawn will usually grow quite fast and will require regular mowing.
Luxury or fine lawn: This is a mix of fine-leaved turfgrasses. These lawns will not tolerate heavy wear and tear, so would not be suitable for children or pets, or areas of high traffic. These mixtures will usually be a blend of chewings fescue, strong and slender creeping red fescue and browntop. The resulting lawn will be very fine in appearance, slow growing and you'll be able to mow at a low height.
Shady lawn: These lawns will tolerate light to medium shade, such as under trees and next to fences or hedges. Most of these mixtures contain hard fescue, strong and slender creeping red fescue and browntop, which are all fine-leaved species. So shady lawns are usually not very hard-wearing.
There are many other mixtures for sale promising 'quick' lawns, 'drought tolerant' lawns, 'easy' lawns and so on, but have a look at the ingredients, and they will usually be very close to those mentioned above.
It goes without saying that you always get what you pay for, and the same goes for grass seed. Unfortunately cheap seed blends are often contaminated with weeds, or contain coarse agricultural perennial ryegrass, not turf ryegrass, meaning you'll get a lawn more suitable for grazing sheep than family barbeques. All you can do to prevent this is by choosing a reputable brand, and reading the ingredients, making sure any perennial ryegrass included is 'turf' or 'fine' ryegrass.