1. polytunnel - keeps nice and cosy warm inside, so some stuff can be started off earlier, overwintered and warmer climate crops (such as sweet potatoes) can be grown here. Not vital, but is a nice addition to your growing experience
2. Greenhouse - starts seeds off early (especially if heated) and is good for warmer climate plants such as chillies and toms, cucmbers and aubergines
3. cold frames - same as above but obviously unheated, warms the ground up earlier and is useful for starting seeds
4. cloches (
) - for putting over frost tender plants and for warming the ground up
cold frame, GH and cloches are probably most important, followed by polytunnel which is a bit of a luxury (for me anyway)