Tribute bands are one dimensional in approach, whereas Wedding Bands and Pub Bands are not. They have the freedom to be able to play a lot more when it comes to choice of musicians and bands. But, whilst they may seem to be very similar in approach there are actually many subtle differences between pub bands and wedding bands.
The other thing with wedding bands is that they tend to be a lot smarter dress code wise. They are playing at somebodies wedding after all and be getting aid. They will appear on photos, be almost representing the feel of the day from the bride and groom's perspective and more than likely be earning a reputation where they will be re-booked by guests - so, the last thing they should be doing is looking like a bunch of tramps.
In contrast to a wedding band, a pub band has a lot more freedom. They can dress much more casually and they can also have a very different kind of set-list to wedding bands. Many pubs do not have dance floors and so it isn't as important to be providing those floor-filling classics, which is why many pub bands will perform tracks by Jimi Hendrix, Cream, Eric Clapton, etc. where the focus is primarily on the musicianship as opposed to the 'getting everyone on their feet' kind of tone.
Pub bands can also afford to be a little more talky with the punters because it isn't about getting people on the dance floor and then keeping them there (like it is with weddings). So, with pub bands you can expect a bit of banter, which again, is great practice for those pub bands that then go onto the wedding scene and teaches them how to deal with the 'odd' drunk guest.
Anyway, hope that clarifies things a bit with regards the differences between Tribute Bands, Wedding Bands and Pub Bands, but if there's anything I've missed please do comment and let me know.
Muchos gracias for reading :)