There are two types of net in the pond world - 'cover nets' and 'fishing nets'.
Cover Nets are the big nets you stretch over your pond in autumn to stop leaves blowing into the water, or to protect your fish from predators. They tend to come in a pack with some pegs to anchor them. Try to keep them taut and don't let them sag into the water as wildlife can get tangled and leave a tunnel or gap at the edge so animals can still visit and drink safely.
Fishing Nets are (you've guessed it!) for scooping leaves,debris or fish out of the pond. Nets can come in all shapes and sizes for different jobs. Generally you have long handles and short handles (so choose based on how far you need to be able to reach) and large heads or small heads. The head shape can be rounded or square - usually square heads are best for dredging along the bottom or skimming floating debris off the surface. The net mesh can be coarse or fine. Coarse works best for catching fish as there is less resistance or 'drag' so you can move the net quicker to catch the little blighters! Fine would be a better choice for skimming duckweed or small bits of floating debris.
If you just need something cheap-and-cheerful, you can't beat the Kiddie Nets - just like you'd find at the seaside! The nylon mesh is not UV stable and they won't last more than a season if stored outside, but for £2.99, Hakuna Matata... The Betta nets are a really good compromise between cost and quality - the mesh is long-lasting and the aluminium handles are sturdy. If you need something extra-long and extra sturdy, you can't beat the JBL net: it has a reinforced edge that doesn't get scuffed while you're dredging sludge, and nice comfy foam grips on the telescopic handle. The OASE Profi Fish Net is also telescopic and very durable, and has the added bonus of coming with a spare net head.
Top Tip! After use, clean your net in tap water and then store it somewhere dry like the shed or garage to make it last longer: being out in the elements takes its toll on even the most durable net.