In considering what it means for some act to be legal or illegal, it helps to distinguish levels of prohibition and entitlement. (As ever, this applies in england and wales, but should be portable to other English Law jurisdictions).

Prohibitions:

Criminal
Doing this act will leave you liable to criminal penalties. This may mean things like prison and criminal records.
Civil liability
Someone in particular can sue you. Depending on the case in question you could be fined, ordered to do (or not do) something, or have property confiscated. If you do insist on doing this kind of thing, consider:
  • How likely is the party to sue you (consider relative costs, bloody-mindedness, and anything else you think relevant)?
  • What kind of cost could you suffer if you fail? Will it be cheaper to just take the consequences?
  • How likely are the courts to want to award exemplary damages if they found out you thought paying up would be cheaper?
Not specifically allowed
This may be a legal grey area, but more likely than not, if someone is claiming that something in this category is "illegal", then they are bullshitting you.
Law is ambiguous
You could potentially be facing civil or criminal liability, but insufficient precedent exists to decide what the law is; this is usually because prosecutions are usually pointless and/or really really hard. The canonical example is home duplication of copyright materials: While english statute has no doctrine of fair use, it is probably illegal to do things like tape your CD's for use in the car, rip mp3's for use in you Archos, and so on. But there is at least one authority supporting fair use; the judiciary might be persuaded to create such a doctrine; furthermore, the penalties excated might be slight
Entitlement:
Absolute entitlement
You can do this. No-one can stop you. You can fuck anyone up who stops you, criminal style, and quite probably civilly as well.
Civil entitlement
You can do this, but any abrogation would only allow you to take civil action. Pushing the matter except in a court may be criminal. Example: Contractual right.
Not prohibited
You are not actually prohibited from doing this. Anyone stopping you may or may not be legally able to do this. Examples include trying to stop you from entering a public place such as a shop (assuming that neither of you have a property right in the land); if they use such tactics as blocking your path, then there is very little you can do about it (other than alert the owner); if they try to stop you violently, then you have recourse against them for their violence