As photographers, we often forget about monopods. Selfie sticks are all the rage now, but what I mean by a monopod specifically, is a single pole your camera can mount on to provide stability when doing photoshoots. Let's say you have to crouch down to get that perfect angle for a shot. Staying in that position while holding your camera can be a pain and you might not get the greatest shots over a long period of time. So, consider using a monopod! It can fit in places where tripods sometimes can't.
There's a wide range of options when you're shopping for monopods. I've even seen someone use their umbrella flipped upside down to provide stability for their camera during a shoot! For an actual single pole device that can stand up straight on its own (mostly) and hold your camera as well, you'll have to buy something. The cheapest models have a little spring button lock to hold sections of the monopod leg out. These can be quite a nuisance to fiddle with, and sometimes, very difficult to unlock. About the only time they unlock reliably is when you're using the lightest of cameras. The concept that you can simply pull the monopod sections out and have everything click into place is better in theory than in reality.
Moving up in price point will get you more reliable functionality. This includes monopods made by reputable companies like Manfrotto and Giottos. These can come with the cam-lever type locks or the twist-to-tighten locks. The cam-action locks seem to be easier to lock and unlock, but can be intrusive with the cam-lever hanging out just waiting to be snagged and released.
The twist locks are much more streamlined, but can be a bear to release if over-tightened (like when trying to hold too heavy a load for the monopod's rating) or if poorly designed, which is what you'll get with a cheaper, off-brand monopod.
The higher end monopods can come with either a long-handled tilt-pan head like on the cheaper tripods or video pods, an inexpensive ball-head, or a 1/4" bolt for screwing directly into your camera's tripod mount or attaching whatever head you'd like to use (at an additional cost).
If you want to go high end with monopods, the sky's the limit. High end varieties can have ball-feet, suction cup feet, snow-spike feet, and the very popular 3-mini legs that make it closer in stability to a tripod.
Monopods can be made from everything from lightweight aluminum to space age plastics. Weight of the monopod can be an important consideration for those that will be doing a lot of hiking to their photoshoot destinations. Speaking of hiking, there are even walking sticks with a 1/4"-20 thread on top for use as a non-collapsible monopod.
The length of the monopod from fully collapsed to fully extended will have to be chosen with your particular needs in place. Does it need to fit in a pocket, backpack or suitcase? Does it need to extend to 9-feet so you can do aerial photography without leaving your seat in the bleachers? Usually, the more sections a monopod has, the greater the difference between collapsed and extended lengths.
Overall, you should buy a monopod that's rated for a little bit more weight than you intend to use so you have some margin should you tend to abuse it. A monopod is a great way to improve your photography's sharpness by holding it all steady and is quite often allowed where tripods are restricted.