It is an opaque market segment to outsiders, and insiders are generally more technologically competent than investment-savvy. This has led to some mushrooming speculation and a cloud of confusion. Not only that, but cryptocurrency markets are unregulated, subject to DDOS and market manipulation, and largely inscrutable.
Fundamentally, "currency" is a marker in an economic system for the exchange of value. Every currency system you're familiar with is fiat currency, which means a central authority dictates what that currency is worth based on how much of it there is. The central authority can print more money or destroy money.
Cryptocurrency can be either commodity currency, where it's worth an actual thing, or representative money, where it can be redeemed for an actual thing.
"Value" in these different currencies comes from what they can be exchanged for. A dollar buys a loaf of cheap bread at the store near me. A Zimbabwean dollar, were I to exchange it, wouldn't buy a crumb. Once upon a time, one bitcoin wouldn't buy a gumball out of a machine. Now it'll buy you a jetski.
Much of that was rudimentary. Here's where it gets interesting:
Bitcoin is a distributed ledger system. Every node on the network has a copy of this ledger and every time transactions are made on this network, those transactions are recorded on every node. It works like bittorrent except everyone has a full copy. A bitcoin - the fundamental marker on the bitcoin network - is worth a fraction of all available markers on the network. In order to incentivize people to participate on the network, there is a reward for mining - these computers work arbitrary problems to be rewarded with bitcoins. This increases the availability of the network, therefore the network effect, therefore the usefulness of the currency network. All of this is controlled by the algorithm, that algorithm subject to change by voting by all participating nodes. It's a digital democracy burning power to keep what's essentially a list of transactions highly available to anybody who wishes to use it.
So. Bitcoin: peer network of machines with a copy of a list of transactions, who get markers to use on that list for being available to record more transactions. Value of Bitcoin is related to what people are willing to pay to use the network. Use of bitcoin is pretty much in value exchange. Bitcoin is representative money. One BTC= one redeemable thing, that redemption being whatever the exchange rate is.
Ethereum is a distributed computing algorithm. Every node on the network can execute code. The algorithm is Turing-complete - this basically means that the algorithm is complex and featured enough to run a copy of itself on itself. So while Bitcoin is a marker, Ethereum is software - it'll do all sorts of stuff. Nobody is entirely sure what all can be done on it yet. However, you can run a browser on it. As a crude example, if you've got a node, Zipcar has a node, the highway department has a node, the gas station has a node, and the grocery store has a node, you could walk out of your house, into a car, drive it over a toll bridge, fill up the tank, grab groceries and go home without ever taking out your wallet as all participants in the network negotiate the value of all transactions. Hell, the car could decide which gas station to go to, and pick a speed based on how much you were willing to bid to get there first. But this is just the crude first look. Probably nobody envisioned Grindr when HTML was laid out. Frankly, a number of cryptocurrencies have been created that run on Ethereum.
So. Ethereum: peer network of machines that run mutual software that allows for contracts, simulations, most anything you can think of. Ether, the unit of measure on Ethereum, is effectively clock cycles on that computer.
When you invest in Bitcoin, you are speculating on the future market value of markers within that currency system. When you invest in Ether, you are speculating on the future market value of processes run on the Ethereum network.