Hi John,
Must agree with most of what has been said, but thought I would also add a few bits.
Carrots, parsnips, potatoes and other roots tend to be planted straight into the bed where they will grow. This is because they
1) are quite hardy and
2) don't like being transplanted.
Tomatoes, courgettes, cucumbers and other tender plants are usually started in trays / pots and then planted out after the last frost. This is because they need protecting early on, and need as much summer as possible to get the most out of them.
Cabbage, broccoli, cauliflowers and any other brassicas can be sown direct, unless your site has club root (ask the other plot holders). If there is club root on site, it is better to plant them when they have a healthy root system. They might still be affected, but have more chance of growing though it.
Hope this helps .....
:-* Tricia