a normal tx is one that has to be signed by 1 person.
a 'contract' is a tx with extra conditions.
for instance
1. a tx required to be signed by more than one person to be deemed valid, where the value being sent to the destinations gets agreed on by all required signers.
2. a tx where the value is only spendable at a certain event.
an example of (2) is the blockreward can only be spent after 100 block confirmations.
an example of (1) is the multisig can only be spent if signed by more than one person.
Lightning network for instance utilises a few of these contract conditions.
a channel is in simple terms just a multisig. a tx which 2 people need to agree on who deserves what amount and both sign. it also has other conditions in that contract that the funds can only be spent after X date/blockheight. and a further condition that 1 party can revoke the other party in certain circumstance.
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