There are many methods of Time Travel, one of which is using a big device full of cool sprockets, pistons, and other gadgets to do the job. A time machine is such a thing. You really have no business fucking with time, causing all manner of paradoxes and time loops.
A time machine is a large device that typically contains a seating area, some nonspecific lever, and may or may not have a display showing when or where you're going. To use the time machine, one will sit, stand, lay, or kneel inside and activate it. After this the user will be transported to some point on their Timeline. The specifics of what happens to the time machine, where the user is taken, and what potential consequences depend on what time machine is refered to.
Some time machines are a car, some time machines are a phone booth, some time machines are a weird baroque contraption that spins around and shoots flames for some reason. The most common aspect of all of these is that they travel through time.
These are time machines that send a user to a point on a timeline, but the machine remains locked in its current position. This is a major disadvantage to any users who wish to go to a point in time and then come back, because they'll have to build a completely new (old) time machine.
Whomever had the bright idea to make two linked Time Transporters wins the prestigious award of duh. Simply put, it's two transporters in two different time periods linked together through some techno wizardry that can be used to travel back and forth between the two transporters. While much better than flinging yourself into some fucking dumbass timeline, the major disadvantage to this is that you only have the option of traveling back and forth between the same two paralel points in time.
It's hard to explain, but a nexus is a time machien that just exists in all of time. You can use this to travel to any point in time and it will be there, ready to take you into its warm, loving arms that are mechanical and cold and probably not arms at all. However, it remains in the same place and can't be moved, so you only have one location, if you travel to a point in time where the place the nexus is is blown to smithereens, you'll fall to your death if you walk out of it.
This is the ultimate time machine in terms of versatility and style. You can hop in this sucker and travel to any point in time and any location. Also this guy is portable, but can be stolen by anyone. Be wary. It's usually fragile and causes whomever uses it mechanical difficulties which is the catalyst for a lot of really whacky adventures.
Just as capable as a time vehicle, the big selling point being that it is a livable building in which you can use to travel through time and space. It's not portable though, because it's a fucking BUILDING. Also these are lame because Dr Who is a terrible show for dumb dickless baby retards who don't have any sense of taste or intellect.