A trifling contract might be considered ultimately unenforceable under contract law because contracts must involve the exchange of consideration in order to be considered valid. A trifling contract, then, would be a contract in which the items involved or exchanged in the contract are not of commensurate value, or are not of any value. Such a contract would likely be deemed trifling and would be considered unenforceable.
An indeterminate contract is a contract in which the terms are unclear or vague. This might include such terms as the parties involved in the contract, the timespan over which the contract functions, and the exact nature of the transaction or exchange involved in the contract. An indeterminate contract might also be enforced, but only in the disfavor of the party which included the indeterminate clauses.
An impossible contract is a contract which cannot possibly be fulfilled due to some condition or circumstance which stands in the way of the fulfillment of the contract. For example, an impossible contract would be a contract to paint a house which has burned down. This means that perfectly enforceable and legal contracts can become unenforceable and impossible through a turn of events.
An illegal contract is a contract which violates some law in its operation or which is designed for a purpose which is considered illegal. A contract to assist with the operation of a laboratory that produces illegal drugs, for example, would ultimately be an illegal contract. Illegal contracts are unenforceable under the law, which means that the parties involved in illegal contracts cannot be held to those terms.