Brown marks can be
1) corking
2) scars
3) infections
4) pests (scale insects)

The marks on your plant do not look dangerous, as far as I can tell. It is not a cactus BTW, but some sort of Euphorbia.

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Looking at the full shot, it appears to be something that happened when the plant was smaller and then it was just the top that was damaged... as the growth above that is fine. Did it get wet one night that got a bit cold?
It looks like cold damage.
If it's not new damage and the top IS growth since, I'd just watch it for any increase.
The darker areas along the ribs look like they could be soft and the start of rot. I'd just watch it for now.
You may have to cut that section out, but then you'll have the top piece to grow and the bottom stump will send out new arms.
But there's many on here with MUCH more experience than me. Hopefully someone else can give you a better diagnosis or confirm mine.
And yeah, it's a Euphorbia, not a cactus.

It is the way it was brought, which was about 2-3 weeks ago, however I'm in the uk, and the day it was brought it was likely below freezing and it was in the car for a fair few hours before I recieved it at home. It was brought from a greengrocers! (I know, random!) so I dont really know how well it's been cared for.

The dark brown splodges aren't soft at all to the touch, and when it was brought there was the smallest of green shoots at the top (the cold seemed to kill these off again).

I've now got it in my "office" sitting on top of one of my computers (which is as you see it in the picture). The temp in here is far more stable than the rest of the house, and so I think that'll hopefully help it recover.

Is there any reason why the bottom is so narrow? It wobbles slightly in the pot and I think this is because it has been replanted recently - the soil isn't very compacted at all. I'm wondering if it needs a different pot also, but I'll look at getting it healthy/growing first.

Could be physical damage too, you know. It's possibly they shipped it with other cacti or spinier euphorbia and it just got jabbed a bunch. Those kinda look like puncture marks to me.

I don't think it needs another pot and it is probably wobbly either because of being newlly planted or because it has no roots at all. Narrowness is normal, particularly for those Euphorbias as they tend to start out smaller when younger.. I think almost all of mine are narrow at the bottom... some will thicken and even form a woody trunk with age and increased stresses (like wind and extreme weight). If no roots yet (these are commonly sold as cuttings and then are very wobbly) just avoid knocking it over and keep it worm. Might be sort of nice to know if there are roots or not as you probably don't want to water it much, or at all, if there are no roots yet. Outdoors here in Southern California that really wounldn't matter as they form roots so quickly and water rarely hurts anything unless the soil is heavy and water-retentive. But I don't know about indoors... never grown one that way.

dj,
As others have mentioned, this is a Euphorbia, probably E. ingens. It will eventually grow into a tree!

I'm guessing that the wobbliness is due to it being planted in a peat-based soil, not because it's lacking roots. Peat-based soils shrink when they're dry...so I would recommend that you replant it in better soil (any cactus or succulent mix will be fine.)

Your plant is a stem cutting, as opposed to a seedling. So it thinks it's pretty old, as old as the plant it was cut from, and wants to start becoming a tree...that's why it has the brownish/yellow spots.

The other brown areas on the stem are a bit more disconcerting.

Keep in mind that in the long run, your plant won't be happy on top of your computer. It may not be a cactus, but it needs just as much sun.

-R